[4.1.x] Fixed #32749 -- Doc'd PyMemcacheCache defaults.

Backport of bb2c5f69f4 from main

docs/topics/cache.txt

@@ -149,6 +149,15 @@ on the IP addresses 172.19.26.240 (port 11211), 172.19.26.242 (port 11212), and
         }
     }
 
+By default, the ``PyMemcacheCache`` backend sets the following options (you can
+override them in your :setting:`OPTIONS <CACHES-OPTIONS>`)::
+
+    'OPTIONS': {
+        'allow_unicode_keys': True,
+        'default_noreply': False,
+        'serde': pymemcache.serde.pickle_serde,
+    }
+
 A final point about Memcached is that memory-based caching has a
 disadvantage: because the cached data is stored in memory, the data will be
 lost if your server crashes. Clearly, memory isn't intended for permanent data