hmp/migration: Fix "migrate" command's documentation

hmp/migration: Fix "migrate" command's documentation

Peter missed the Sphinx HMP document for the "resume/-r" flag in commit
7a4da28b26 ("qmp: hmp: add migrate "resume" option").  Add it.

When at it, slightly cleanup the lines around:

  - Move "detach/-d" to a separate section rather than appending it at the
  end of the command description. Add a hint for how to query the migration
  results in detached mode.

  - Add "postcopy" keyword to "resume/-r" help messages, as it only applies
  to postcopy.

Cc: Dr. David Alan Gilbert <dave@treblig.org>
Cc: Fabiano Rosas <farosas@suse.de>
Fixes: 7a4da28b26 ("qmp: hmp: add migrate "resume" option")
Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>