pgsql: Clean up some misplaced comments in partition_join.sql regressio

Clean up some misplaced comments in partition_join.sql regression test.

Also, add a comment explaining a test case.

Back-patch to 11 where the regression test was added.

Discussion: /messages/by-id/CAPmGK15adZPh2B+mGUjSOMH+H39ogDRWfCfm4G6jncZCAs9V_Q@mail.gmail.com

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e3ac8939324d73c35e2fd0f846001eb02e3a7858

Modified Files
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src/test/regress/expected/partition_join.out | 10 ++++++----
src/test/regress/sql/partition_join.sql | 11 +++++++----
2 files changed, 13 insertions(+), 8 deletions(-)