Add further commentary about synchronize process_remote. (#16292)

Addresses #16284
(cherry picked from commit dc9b53a)

lib/ansible/plugins/action/synchronize.py

@@ -85,8 +85,15 @@ def _process_remote(self, host, path, user, port_matches_localhost_port):
             alternative ssh port to a vagrant host.)
         """
         transport = self._connection.transport
+        # If we're connecting to a remote host or we're delegating to another
+        # host or we're connecting to a different ssh instance on the
+        # localhost then we have to format the path as a remote rsync path
         if host not in C.LOCALHOST or transport != "local" or \
                 (host in C.LOCALHOST and not port_matches_localhost_port):
+            # If we're delegating to non-localhost and but the
+            # inventory_hostname host is localhost then we need the module to
+            # fix up the rsync path to use the controller's public DNS/IP
+            # instead of "localhost"
             if port_matches_localhost_port and host in C.LOCALHOST:
                 self._task.args['_substitute_controller'] = True
             return self._format_rsync_rsh_target(host, path, user)