qapi: Tidy up a slightly awkward TODO comment

MigrateSetParameters has a TODO comment sitting right behind its doc
comment.  I wrote it this way to keep it out of the manual, but that
reason is not obvious.

The previous commit (sphinx/qapidoc: Do not emit TODO sections into
user manuals) lets me move it into the doc comment as a TODO section.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230428105429.1687850-8-armbru@redhat.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

qapi/migration.json

@@ -945,10 +945,11 @@
 # Features:
 # @unstable: Member @x-checkpoint-delay is experimental.
 #
+# TODO: either fuse back into MigrationParameters, or make
+#       MigrationParameters members mandatory
+#
 # Since: 2.4
 ##
-# TODO either fuse back into MigrationParameters, or make
-# MigrationParameters members mandatory
 { 'struct': 'MigrateSetParameters',
   'data': { '*announce-initial': 'size',
             '*announce-max': 'size',