Fix comments for ChangeVarNodes() and related functions

 Fix comments for ChangeVarNodes() and related functions
 
 The comment for ChangeVarNodes() refers to a parameter named
 change_RangeTblRef, which does not exist in the code.
 
 The comment for ChangeVarNodesExtended() contains an extra space,
 while the comment for replace_relid_callback() has an awkward line
 break and a typo.
 
 This patch fixes these issues and revises some sentences for smoother
 wording.
 
 Oversights in commits ab42d643c and fc069a3a6.
 
 Author: Richard Guo <guofenglinux@gmail.com>
 Discussion: https://postgr.es/m/CAMbWs480j16HC1JtjKCgj5WshivT8ZJYkOfTyZAM0POjFomJkg@mail.gmail.com
 Backpatch-through: 18