x11: document the latest best advice about graphics

Recommend PORTS_MODULES for drm and nvidia ports. Remove pkg suggestion because
it doesn't work in too many cases. Provide example for make.conf.

Comments from: adridg@, Juan Molina, rpokala@, greg at unrelenting technology, khng,
and others on sparc64 and twitter
Differential Revision: https://reviews.freebsd.org/D29479