pgsql: Improve commentary about ChangeVarNodesWalkExpression().

Improve commentary about ChangeVarNodesWalkExpression().

IMO the proximate cause of the bug fixed in commit 07b7a964d
was sloppy thinking about what ChangeVarNodesWalkExpression()
is to be used for.  Flesh out its header comment to try to
improve that situation.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1607553.1774017006@sss.pgh.pa.us
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a0e0b3cc685aa58d9ff63ed64e45ac1cba297e40

Modified Files
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src/backend/rewrite/rewriteManip.c | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)