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

[Florent Thoumie]

Oliver Eikemeier wrote:

> 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. :)

	I'm not scared at all :)

	I'll try to dig further into this, and re-submit a PR to
	supersede this one. I'll send one to doc@ too.