Further clarification of $defs/definitions

Introduce the category of reserved location keywords, which
are basically just $defs and $comment.  This should make it
more clear that you can create keywords of your own like this,
and fits them more nicely into the overall keyword taxonomy.

Part of this change refers to the typical core+validation
meta-schema as the "default meta-schema", which is a change
made in another PR.

Author: handrews

jsonschema-core.xml

@@ -307,7 +307,7 @@
                     <t>
                         Object properties that are applied to the instance are called keywords,
                         or schema keywords.  Broadly speaking, keywords fall into one
-                        of three categories:
+                        of four categories:
                         <list style="hanging">
                             <t hangText="assertions:">
                                 produce a boolean result when applied to an instance
@@ -319,6 +319,10 @@
                                 apply one or more subschemas to a particular location
                                 in the instance, and combine or modify their results
                             </t>
+                            <t hangText="reserved locations:">
+                                do not directly affect results, but reserve a place
+                                for a specific purpose to ensure interoperability
+                            </t>
                         </list>
                     </t>
                     <t>
@@ -1004,6 +1008,22 @@
                     </section>
                 </section>
             </section>
+            <section title="Reserved Locations">
+                <t>
+                    A fourth category of keywords simply reserve a location to hold re-usable
+                    components or data of interest to schema authors that is not suitable
+                    for re-use.  These keywords do not affect validation or annotation results.
+                    Their purpose in the core vocabulary is to ensure that locations are
+                    available for certain purposes and will not be redefined by extension
+                    keywords.
+                </t>
+                <t>
+                    While these keywords do not directly affect results, as explained in section
+                    <xref target="non-schemas" format="counter"></xref> unrecognized
+                    extension keywords that reserve locations for re-usable schemas may have
+                    undesirable interactions with references in certain circumstances.
+                </t>
+            </section>
         </section>
         <section title="The JSON Schema Core Vocabulary">
             <t>
@@ -1848,10 +1868,10 @@
                         </t>
                     </section>
 
-                    <section title="References to Possible Non-Schemas">
+                    <section title="References to Possible Non-Schemas" anchor="non-schemas">
                         <t>
                             Subschema objects (or booleans) are recognized by their use with known
-                            applicator keywords, or with placeholder keywords such as
+                            applicator keywords or with placeholder keywords such as
                             <xref target="defs">"$defs"</xref> that take one or more subschemas
                             as a value.  These keywords may be "$defs" and the standard applicators
                             from this document, or extension keywords from a known vocabulary, or
@@ -1977,7 +1997,7 @@
 
                 <section title='Schema Re-Use With "$defs"' anchor="defs">
                     <t>
-                        The "$defs" keyword provides a standardized location for schema
+                        The "$defs" keyword reserves a location for schema
                         authors to inline re-usable JSON Schemas into a more general schema.
                         The keyword does not directly affect the validation result.
                     </t>
@@ -2012,8 +2032,8 @@
 
             <section title='Comments With "$comment"'>
                 <t>
-                    This keyword is reserved for comments from schema authors to readers or
-                    maintainers of the schema.
+                    This keyword is reserved a location for comments from schema authors
+                    to readers or maintainers of the schema.
 
                     The value of this keyword MUST be a string. Implementations MUST NOT present this
                     string to end users.  Tools for editing schemas SHOULD support displaying and

jsonschema-validation.xml

@@ -179,7 +179,8 @@
                 The current URI for the JSON Schema Validation meta-schema is
                 <eref target="http://json-schema.org/draft/2019-08/schema#"/>.
                 For schema author convenience, this meta-schema describes all vocabularies
-                defined in this specification and the JSON Schema Core specification.
+                defined in this specification and the JSON Schema Core specification,
+                as well as two former keywords which are reserved for a transitional period.
                 Individual vocabulary and vocabulary meta-schema URIs are given for
                 each section below.  Certain vocabularies are optional to support, which
                 is explained in detail in the relevant sections.
@@ -1352,7 +1353,11 @@
                         Renamed to "$defs" to match "$ref" and be shorter to type.
                         Schema vocabulary authors SHOULD NOT define a "definitions" keyword
                         with different behavior in order to avoid invalidating schemas that
-                        still use the older name.
+                        still use the older name.  While "definitions" is absent in the
+                        single-vocabulary meta-schemas referenced by this document, it
+                        remains present in the default meta-schema, and implementations
+                        SHOULD assume that "$defs" and "definitions" have the same
+                        behavior when that meta-schema is used.
                     </t>
                     <t hangText='"allOf", "anyOf", "oneOf", "not", "if", "then", "else",
                                  "items", "additionalItems", "contains", "propertyNames",