migration/qmp: Update "resume" flag doc in "migrate" command

It wasn't obvious how the resume flag should be used when staring at the
QAPI doc.  Enrich it to be crystal clear.

Reported-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Link: https://lore.kernel.org/r/20251022190425.2730441-1-peterx@redhat.com
[peterx: amended wordings, per markus]
Signed-off-by: Peter Xu <peterx@redhat.com>

qapi/migration.json

@@ -1732,7 +1732,10 @@
 # @detach: this argument exists only for compatibility reasons and is
 #     ignored by QEMU
 #
-# @resume: resume one paused migration, default "off".  (since 3.0)
+# @resume: when set, use the new uri/channels specified to resume paused
+#     postcopy migration.  This flag should only be used if the previous
+#     postcopy migration was interrupted.  The command will fail unless
+#     migration is in "postcopy-paused" state.  (default: false, since 3.0)
 #
 # Features:
 #