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

[Oliver Eikemeier]

Florent Thoumie wrote:
> 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.

I totally agree that PORTDOCS is underdocumented and should be
explained more clearly. I appreciate efforts and didn't mean to
scare you away. Please add a, additional patch for the Porter's
Handbook. :)

-Oliver