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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master

Details
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https://git.postgresql.org/pg/commitdiff/360dd6f7b4c522d9a48d4621feb6da0a91bd7594

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