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>

hmp-commands.hx

@@ -912,14 +912,20 @@ ERST
         .args_type  = "detach:-d,resume:-r,uri:s",
         .params     = "[-d] [-r] uri",
         .help       = "migrate to URI (using -d to not wait for completion)"
-		      "\n\t\t\t -r to resume a paused migration",
+		      "\n\t\t\t -r to resume a paused postcopy migration",
         .cmd        = hmp_migrate,
     },
 
 
 SRST
-``migrate [-d]`` *uri*
-  Migrate to *uri* (using -d to not wait for completion).
+``migrate [-d] [-r]`` *uri*
+  Migrate the VM to *uri*.
+
+  ``-d``
+    Start the migration process, but do not wait for its completion.  To
+    query an ongoing migration process, use "info migrate".
+  ``-r``
+    Resume a paused postcopy migration.
 ERST
 
     {