pgsql: Update comment about set_join_pathlist_hook().

Update comment about set_join_pathlist_hook().

The comment introduced by commit e7cb7ee14 was a bit too terse, which
could lead to extensions doing different things within the hook function
than we intend to allow.  Extend the comment to explain what they can do
within the hook function.

Back-patch to all supported branches.

In passing, I rephrased a nearby comment that I recently added to the
back branches.

Reviewed by David Rowley and Andrei Lepikhov.

Discussion: https://postgr.es/m/CAPmGK15SBPA1nr3Aqsdm%2BYyS-ay0Ayo2BRYQ8_A2To9eLqwopQ%40mail.gmail.com

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a3a66d3ff18c5a4b49a9fbebce55aa83711953f6

Modified Files
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src/backend/optimizer/path/joinpath.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)