Follows-up f36b73e96c, which moved these classes to libs/objectcache.
* Fix wrong @throws in MemcachedPeclBagOStuff.
* Fix wrong @returns in MemcachedBagOStuff::getClient().
* Rename MWMemcached to MemcachedClient.
* Remove mention of 'memcached.php', which doesn't exist anymore.
Change-Id: I34dbc859be4778cea489fd2344f233f30452605c