Update locking documentation

* added how it works in clustered caches

Explained in the documentation how the locking works for clustered caches.

Dan, let me know what you think.
For conditional commands, the validation still happens on the originator. In addition to that, the write skew check is done for any command for which the transaction read the previous value (e.g. with an explicit get operation, with a conditional operation, or with a regular put operation that returned the previous value).