qapi: remove trivial "Returns:" sections

qapi: remove trivial "Returns:" sections

The new qapidoc.py can generate "Returns" statements with type
information just fine, so we can remove it from the source where it
doesn't add anything particularly novel or helpful and just repeats the
type info.

This patch is fairly "gentle" and doesn't aggressively touch other
"Returns" lines that could be rephrased to omit repeating type
information; it only removes lines that appear appropriate to wholly
remove.

To help facilitate auto-generated placement, a few doc blocks have a
"TODO:" line inserted to help the placement algorithm differentiate the
introductory paragraph(s) from the rest of the documentation.

The auto-generated returns are in the exact same spot, except for
query-migrationthreads, query-machines, and
x-query-virtio-queue-element.  These auto-generation moves to a better
spot.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250711051045.51110-4-jsnow@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Two more Returns: removed, commit message amended to explain
auto-generated returns generated into a different spot]
Signed-off-by: Markus Armbruster <armbru@redhat.com>