docs/devel/qapi-code-gen: Belatedly update features documentation

Commit 013b4efc9be "qapi: Add feature flags to remaining
definitions" (v5.0.0), commit 84ab0086879 "qapi: Add feature flags to
struct members" (v5.0.0), and commit b6c18755e41 "qapi: Add feature
flags to enum members" (v6.2.0) neglected to update section
"Features".  Make up for that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230213132009.918801-2-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

docs/devel/qapi-code-gen.rst

@@ -685,9 +685,10 @@ change in the QMP syntax (usually by allowing values or operations
 that previously resulted in an error).  QMP clients may still need to
 know whether the extension is available.
 
-For this purpose, a list of features can be specified for a command or
-struct type.  Each list member can either be ``{ 'name': STRING, '*if':
-COND }``, or STRING, which is shorthand for ``{ 'name': STRING }``.
+For this purpose, a list of features can be specified for definitions,
+enumeration values, and struct members.  Each feature list member can
+either be ``{ 'name': STRING, '*if': COND }``, or STRING, which is
+shorthand for ``{ 'name': STRING }``.
 
 The optional 'if' member specifies a conditional.  See `Configuring
 the schema`_ below for more on this.