DOCS-1318 change text for mongodb operations synonymous to alter table

source/includes/table-sql-to-mongo-schema-examples.yaml

@@ -59,19 +59,41 @@ sql2: |
          ALTER TABLE users
          ADD join_date DATETIME
 mongo2: |
-        Collections do not describe or enforce the structure of the
-        constituent documents. See the :doc:`/core/data-modeling`
-        page for more information.
+        Collections do not describe or enforce the structure of its
+        documents; i.e. there is no structural alteration at the
+        collection level.
+
+        However, at the document level, :method:`update()
+        <db.collection.update()>` operations can add fields to existing
+        documents using the :operator:`$set` operator.
+
+        .. code-block:: javascript
+           :emphasize-lines: 1-5
+
+           db.users.update(
+               { },
+               { $set: { join_date: new Date() } },
+               { multi: true }
+           )
 ref2: |
-      See :method:`update() <db.collection.update()>` and
-      :operator:`$set` for more information on changing the structure
-      of documents in a collection.
+      See the :doc:`/core/data-modeling`, :method:`update()
+      <db.collection.update()>`, and :operator:`$set` for more
+      information on changing the structure of documents in a
+      collection.
 sql3: |
       .. code-block:: sql
 
          ALTER TABLE users
          DROP COLUMN join_date
 mongo3: |
+        Collections do not describe or enforce the structure of its
+        documents; i.e. there is no structural alteration at the
+        collection level.
+
+        However, at the document level, :method:`update()
+        <db.collection.update()>` operations can remove fields from
+        documents using the :operator:`$unset` operator.
+
         .. code-block:: javascript
            :emphasize-lines: 1-5
 
@@ -81,8 +103,10 @@ mongo3: |
                { multi: true }
            )
 ref3: |
-      See :method:`update() <db.collection.update()>` and
-      :operator:`$unset` for more information.
+      See :doc:`/core/data-modeling`, :method:`update()
+      <db.collection.update()>`, and :operator:`$unset` for more
+      information on changing the structure of documents in a
+      collection.
 sql4: |
       .. code-block:: sql