go/token: rename RemoveLine to MergeLine, improve documentation

This is a follow-up to feedback from gri in
https://codereview.appspot.com/12837044/. Most of the wording
and naming improvements are lifted shamelessly from him.

R=gri
CC=golang-dev
https://codereview.appspot.com/13670043

Committer: Robert Griesemer <gri@golang.org>