added more `readme.txt` tags

git-svn-id: https://plugins.svn.wordpress.org/redis-cache/trunk@1027316 b8457f37-d9ea-0310-8a92-e5e31aec5664
code cleanup; verify user capability in `do_admin_actions()`; hide admin notices from users without the right capability

git-svn-id: https://plugins.svn.wordpress.org/redis-cache/trunk@1027321 b8457f37-d9ea-0310-8a92-e5e31aec5664