Sentinel doc updates

This is for #404 regarding the `SENTINELS` command, but also adds more docs covering the re-use of a slave in a different master or as a new master (ie. instance re-allocation)

It also discusses how to add a master which has requirepass set through the API and makes minor spelling/formatting/grammar corrections.

... and it looks like I accidentally had my change to clients.json adding libredis in this branch. If you want I can break that out to a separate PR as I didn't realize it was here.