Three changes:
1. mention "move" to clarify things up.
2. use {old,new}path instead of {old,new}name, which makes it clear what
   relative path would do here.
3. mention "OS-specific restrictions might apply".

Fixes #6887.

R=golang-dev, alex.brainman, iant, r
CC=golang-dev
https://golang.org/cl/36930044