pgsql: 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(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs480j16HC1JtjKCgj5WshivT8ZJYkOfTyZAM0POjFomJkg@mail.gmail.com
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/59dec6c0b09f717e4ce78d54f578d56ef93bcc53

Modified Files
--------------
src/backend/optimizer/plan/analyzejoins.c | 18 +++++++++---------
src/backend/rewrite/rewriteManip.c | 19 ++++++++-----------
2 files changed, 17 insertions(+), 20 deletions(-)