kde.mk: Support KDE ports with no DOCS

The kde.mk framework automatically does DOCS for KDE ports --
but not all of them have documentation, so then you get a
weird situation of OPTIONS_DEFINE=DOCS in a port that has
no documentation (e.g. sysutils/baloo-widgets) where the
option has no effect at all.

Massage the machinery a little so that the bogus option can
be dropped; KDE ports with no docs should not define a DOCS
option any more.

PR: 252363