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://golang.org/cl/
146520043