Clarify CommonMark extensibility.

Author: handrews

versions/3.0.4.md

@@ -211,7 +211,7 @@ Note that the encoding indicated by `byte`, which inflates the size of data in o
 
 ### <a name="richText"></a>Rich Text Formatting
 Throughout the specification `description` fields are noted as supporting CommonMark markdown formatting.
-Where OpenAPI tooling renders rich text it MUST support, at a minimum, markdown syntax as described by [CommonMark 0.27](https://spec.commonmark.org/0.27/). Tooling MAY choose to ignore some CommonMark features to address security concerns. 
+Where OpenAPI tooling renders rich text it MUST support, at a minimum, markdown syntax as described by [CommonMark 0.27](https://spec.commonmark.org/0.27/). Tooling MAY choose to implement extensions on top of CommonMark 0.27, and MAY choose to ignore some CommonMark or extension features to address security concerns. 
 
 ### <a name="relativeReferences"></a>Relative References in URLs