REPL doc command enhancements

[som-snytt]

Please just show me the doc for an identifier. "Look it up".

scala> :doc Selectable
// No doc for `Selectable`

scala> :doc new Selectable
A marker trait for objects that support structural selection via
  `selectDynamic` and `applyDynamic`

Maybe needs a separate feature request, but show me docs for implicit scope!

First, here I want to see doc for companion pair, then I'd like to see the doc for the givens.

scala> :doc CanEqual
Companion object containing a few universally known `CanEqual` instances.
  CanEqual instances involving primitive types or the Null type are handled directly in
  the compiler (see Implicits.synthesizedCanEqual), so they are not included here.



scala> :doc CanEqual.derived
A universal `CanEqual` instance.