spec: remove most ?links for the now more common in-text links

Also:
- more consistenly use "xxx" statement rather than <code>xxx</code> statement
- fix/remove unnecessary links

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7821050
net: fix documentation for UnixAddr

Also simplifies ResolveUnixAddr.

R=golang-dev, dave, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7510047