dns/knot3: Don't abuse DOCS for MANPAGES

The %%DOCS%% substitution was both installing the manpages
and the documentation, so fix that by using MANPAGES instead.

While here:

• Remove option descriptions for IDN and DOCS since they are already defined in Mk/bsd.options.desc.mk.
• Pet portclippy.

PR: 298789
Approved by: osa, vvd (Mentors, implicit)
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>