SI-7376 Scaladoc warns when discarding local doc comments with API tags

Double-star doc comments in non-dockable positions at the end of a block
will emit a warning only if API tags like @author are present, or under
-Xlint.

A real comment parser is applied early to probe for tags, to minimize
ad hoc testing or duplication, but warnings are suppressed. Residual
ad hockiness lies in precisely which tags to warn on.  Ad hoc or ad doc.

This fix is a stop gap; a richer solution would also report about other
doc locations that won't be processed.

Review by @paulp, of whose idea this is a refinement.  Of whom?