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_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/c50ad79f609c8a66eef3e58afd97b6987df59f96

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