[3.1.x] Used :mimetype: role in various docs.

Backport of feb91dbda1 from master

docs/internals/contributing/writing-documentation.txt

@@ -243,6 +243,9 @@ documentation:
   and try to link to the relevant section if possible. For example, use
   ``:pep:`20#easter-egg``` or ``:pep:`Easter Egg <20#easter-egg>```.
 
+* Use :rst:role:`:mimetype:<mimetype>` to refer to a MIME Type unless the value
+  is quoted for a code example.
+
 Django-specific markup
 ======================