go/format, cmd/gofmt: added missing comments, minor internal cleanup

This is a minor cleanup following CL 142360043:

The internal parse and format functions in both packages
were almost identical - made them identical by adding an
extra parameter, and documented them as identical.

Eventually we should find a nice way to factor these functions
out, but we cannot do this now while in prep for 1.4.

No functionality change.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/146520043