pgsql: Improve some comments related to executor result relations.

Improve some comments related to executor result relations.

es_leaf_result_relations doesn't exist; perhaps this was an old name
for es_tuple_routing_result_relations, or maybe this comment has gone
unmaintained through multiple rounds of whacking the code around.

Related comment in execnodes.h was both obsolete and ungrammatical.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/26cb82030fa83e222f5744fb8254ec9073162273

Modified Files
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src/backend/executor/execMain.c | 19 +++++++++++--------
src/include/nodes/execnodes.h   |  9 ++++-----
2 files changed, 15 insertions(+), 13 deletions(-)