FAQ: lots of small tweaks plus a couple of new discussions.

Expand the conversations about pointers, memory, and
garbage collection.
Describe the lack of co/contravariant typing.

Fixes #1929.
Fixes #1930.

R=dsymonds, r, mirtchovski, edsrzf, hanwen, rsc
CC=golang-dev
https://golang.org/cl/4852041