pgsql: Further improve commentary about ChangeVarNodesWalkExpression()

Further improve commentary about ChangeVarNodesWalkExpression()

The updated comment explains why we use ChangeVarNodes_walker() instead of
expression_tree_walker(), and provides a bit more detail about the differences
in processing top-level Query and subqueries.

Author: Alexander Korotkov <aekorotkov@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAPpHfdvbjq342WTQ705Wmqhe8794pcp7wospz%2BWUJ2qB7vuOqA%40mail.gmail.com
Backpatch-through: 18

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/8c73ab9da9f191e07d5e1c11b59e2dcbde8cafb8

Modified Files
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src/backend/rewrite/rewriteManip.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)