Docs: Document how to use observe properly outside of the "wrapped" methods

There will be some cases where a user wants to use the observe functionality outside of the set()/add()/replace() methods what we have allowed for them to specify the "durabililty requirements".

Specifcally this may come up when using incr/decr/prepend/append and wanting to "observe" on that...or just using observe manually to control the behavior, retry, timeouts, etc.