ports/180949: Add some documentation to bsd.options.mk knobs.

[edwin at FreeBSD.org]

Synopsis: Add some documentation to bsd.options.mk knobs.

Responsible-Changed-From-To: freebsd-ports-bugs->portmgr
Responsible-Changed-By: edwin
Responsible-Changed-When: Tue Jul 30 17:10:08 UTC 2013
Responsible-Changed-Why: 
bsd.options.mk is portmgr territory (via the GNATS Auto Assign Tool)

http://www.freebsd.org/cgi/query-pr.cgi?pr=180949