Amend staking docs to account for state of controller deprecation (#2451)

Amends some staking pallet docs, and deprecation comment, to adjust to
the latest controller deprecation state.

Note, do we need the `README.md` file, which is a duplicate of the
pallet docs? Docs would be easier to maintain, and less ambiguity for
devs to refer to, if we had one source of truth in the generated pallet
docs.