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

src/backend/optimizer/plan/analyzejoins.c

@@ -1683,14 +1683,14 @@ add_non_redundant_clauses(PlannerInfo *root,
 }
 
 /*
- * A custom callback for ChangeVarNodesExtended() providing
+ * A custom callback for ChangeVarNodesExtended() providing Self-join
- * Self-join elimination (SJE) related functionality
+ * elimination (SJE) related functionality
  *
- * SJE needs to skip the RangeTblRef node
+ * SJE needs to skip the RangeTblRef node type.  During SJE's last
- * type.  During SJE's last step, remove_rel_from_joinlist() removes
+ * step, remove_rel_from_joinlist() removes remaining RangeTblRefs
- * remaining RangeTblRefs with target relid.  If ChangeVarNodes() replaces
+ * with target relid.  If ChangeVarNodes() replaces the target relid
- * the target relid before, remove_rel_from_joinlist() fails to identify
+ * before, remove_rel_from_joinlist() would fail to identify the nodes
- * the nodes to delete.
+ * to delete.
  *
  * SJE also needs to change the relids within RestrictInfo's.
  */
@@ -1769,8 +1769,8 @@ replace_relid_callback(Node *node, ChangeVarNodes_context *context)
 			/*
 			 * For self-join elimination, changing varnos could transform
 			 * "t1.a = t2.a" into "t1.a = t1.a".  That is always true as long
-			 * as "t1.a" is not null.  We use qual() to check for such a case,
+			 * as "t1.a" is not null.  We use equal() to check for such a
-			 * and then we replace the qual for a check for not null
+			 * case, and then we replace the qual with a check for not null
 			 * (NullTest).
 			 */
 			if (leftOp != NULL && equal(leftOp, rightOp))

src/backend/rewrite/rewriteManip.c

@@ -542,8 +542,6 @@ offset_relid_set(Relids relids, int offset)
  * (identified by sublevels_up and rt_index), and change their varno fields
  * to 'new_index'.  The varnosyn fields are changed too.  Also, adjust other
  * nodes that contain rangetable indexes, such as RangeTblRef and JoinExpr.
- * Specifying 'change_RangeTblRef' to false allows skipping RangeTblRef.
- * See ChangeVarNodesExtended for details.
  *
  * NOTE: although this has the form of a walker, we cheat and modify the
  * nodes in-place.  The given expression tree should have been copied
@@ -667,14 +665,13 @@ ChangeVarNodes_walker(Node *node, ChangeVarNodes_context *context)
  * ChangeVarNodesExtended - similar to ChangeVarNodes, but with an additional
  *							'callback' param
  *
- * ChangeVarNodes changes a given node and all of its underlying nodes.
+ * ChangeVarNodes changes a given node and all of its underlying nodes.  This
- * This version of function additionally takes a callback, which has a
+ * version of function additionally takes a callback, which has a chance to
- * chance to process a node before ChangeVarNodes_walker.  A callback
+ * process a node before ChangeVarNodes_walker.  A callback returns a boolean
- * returns a boolean value indicating if given node should be skipped from
+ * value indicating if the given node should be skipped from further processing
- * further processing by ChangeVarNodes_walker.  The callback is called
+ * by ChangeVarNodes_walker.  The callback is called only for expressions and
- * only for expressions and other children nodes of a Query processed by
+ * other children nodes of a Query processed by a walker.  Initial processing
- * a walker.  Initial processing of the root Query doesn't involve the
+ * of the root Query doesn't involve the callback.
- * callback.
  */
 void
 ChangeVarNodesExtended(Node *node, int rt_index, int new_index,