[COMMITTERS] pgsql: Improve comments for execExpr.c'sisAssignmentIndirectionExpr().

Improve comments for execExpr.c's isAssignmentIndirectionExpr().

I got confused about why this function doesn't need to recursively
search the expression tree for a CaseTestExpr node.  After figuring
that out, add a comment to save the next person some time.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e9b64824a067f8180e2553127467d7522516122c

Modified Files
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src/backend/executor/execExpr.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)