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
------
REL_14_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/c7c4ce2be363ac5cf5141e73fd7966e0bbfe401f

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