qemu-img-cmds.hx: add passive-aggressive note

I'm kidding. It's very easy to forget there are per-command sections
in the texi, and insane that we don't autogenerate those, too.

Until then, leave a little post-it note in this .hx file until I
find a way to delete it.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

qemu-img-cmds.hx

@@ -6,6 +6,9 @@ HXCOMM DEF(command, callback, arg_string) is used to construct
 HXCOMM command structures and help message.
 HXCOMM HXCOMM can be used for comments, discarded from both texi and C
 
+HXCOMM When amending the TEXI sections, please remember to copy the usage
+HXCOMM over to the per-command sections in qemu-img.texi.
+
 STEXI
 @table @option
 ETEXI