ports/67036: bsd.port.mk: Clarify PORTDOCS comment.

[Florent Thoumie]

Talking to Oliver, it seems I misunderstood the role of PORTDOCS,
or at least, I thought it handles more things that it actually
does.

There should be a note telling that PORTDOCS is automatically added
to the package list, I don't think it's in the Porter's Handbook.
It's not a serious problem as it is only a way to reduce pkg-plist
but I think it worth it.