net: document sockaddr interface

This is in preparation for runtime-integrated network pollster for BSD
variants.

Update issue 5199

R=golang-dev, fvbommel, dave
CC=golang-dev
https://codereview.appspot.com/11984043
net: remove redundant comment on isWildcard

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/11985043