Generalize and publicize @deprecated{Inheritance, Overriding}

[scabug]

In 2.10, we've added the ability to signal the impending finality of a class, or a method, issing a deprecation warning to extenders/overriders.

We left this facility private[scala]], as it came quite late in the release cycle, and didn't have much time for design discussions.

Some discussion about what should be added started here: scala/scala#1026

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-6383?orig=1
Reporter: @retronym
See #6162

[dwijnand]

Resolved as of scala/scala#4827 I'd say. Reopen (or comment) if you disagree with me.