lncli: update lncli state description

This updates the description that is provided for `lncli state` to match
the states that are currently returned by lnd. `RPC_READY` no longer
exists and is replaced by `RPC_ACTIVE` with a revised description, and
`SERVER_ACTIVE` is added as the status that indicates lnd is fully
ready to accept RPC calls.

See `enum WalletState` in `stateservice.proto`.

docs/release-notes/release-notes-0.15.0.md

@@ -16,6 +16,8 @@
 * Add [private status](https://github.com/lightningnetwork/lnd/pull/6167)
   to pendingchannels response.
 
+* [Update description for `state` command](https://github.com/lightningnetwork/lnd/pull/6237).
+
 ## Bug Fixes
 
 * [Fixed an inactive invoice subscription not removed from invoice