buffer: docs-only deprecate SlowBuffer

With the addition of `Buffer.allocUnsafeSlow(size)`
`SlowBuffer` can be deprecated... but docs-only for now.

PR-URL: https://github.com/nodejs/node/pull/5833


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>