Commit 6a8c0b51 "qapi: Add feature flags to struct types" neglected
to document how to document feature flags.  Make up for that.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20211026111023.76937-3-armbru@redhat.com>
[Editing accident fixed]