migration: Update migrate_cancel documentation

Update the migrate_cancel command documentation with a few words about
postcopy and the expected state of the machine after migration.

Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Message-ID: <20250213175927.19642-10-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>

qapi/migration.json

@@ -1524,7 +1524,9 @@
 ##
 # @migrate_cancel:
 #
-# Cancel the current executing migration process.
+# Cancel the currently executing migration process.  Allows a new
+# migration to be started right after.  When postcopy-ram is in use,
+# cancelling is not allowed after the postcopy phase has started.
 #
 # .. note:: This command succeeds even if there is no migration
 #    process running.