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>

qapi/qom.json

@@ -637,7 +637,7 @@
 #
 # @discard-data: if true, the file contents can be destroyed when QEMU exits,
 #                to avoid unnecessarily flushing data to the backing file. Note
-#                that ``discard-data`` is only an optimization, and QEMU might
+#                that @discard-data is only an optimization, and QEMU might
 #                not discard file contents if it aborts unexpectedly or is
 #                terminated using SIGKILL. (default: false)
 #