qapi: @foo should be used to reference, not ``foo``

Documentation suggests @foo is merely shorthand for ``foo``.  It's
not, it carries additional meaning: it's a reference to a QAPI schema
name.

Reword the documentation to spell that out.

Fix up the few ``foo`` that should be @foo.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230425064223.820979-7-armbru@redhat.com>