SI-8948 resurrect the scaladoc for Any/AnyRef/Nothing/Null.

They disappeared in dc1cd96 when `scala.tools.nsc.doc.DocParser` lost
its `override def forScaladoc = true`, which used to trigger whether to
parse `DocDef` nodes. This was deprecated in favor of a custom global
when scaladoc was modularized out (#2226), but `DocParser` didn't get it
since it didn't override `forScaladoc` anymore...

I added back `forScaladoc` in `DocParser` for consistency with
`ScaladocGlobal`, although it doesn't do anything anymore.