tests/qapi-schema: Rework comments on longhand member definitions

tests/qapi-schema: Rework comments on longhand member definitions

A few old comments talk about "desired future use of defaults" and
"anonymous inline branch types".  Kind of misleading since commit
87adbbffd4 "qapi: add a dictionary form for TYPE" added longhand
member definitions.  Talk about that instead.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210323094025.3569441-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>