qapi: Fix bogus documentation of query-migrationthreads

The doc comment documents an argument that doesn't exist.  Would
fail compilation if it was marked up correctly.  Delete.

The Returns: section fails to refer to the data type, leaving the user
to guess.  Fix that.

The command name violates QAPI naming rules: it should be
query-migration-threads.  Too late to fix.

Reported-by: John Snow <jsnow@redhat.com>
Fixes: 6713262 (migration: Introduce interface query-migrationthreads)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240322135117.195489-4-armbru@redhat.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

qapi/migration.json

@@ -2419,9 +2419,7 @@
 #
 # Returns information of migration threads
 #
-# data: migration thread name
-#
-# Returns: information about migration threads
+# Returns: @MigrationThreadInfo
 #
 # Since: 7.2
 ##