pgsql: Fix outdated comments in slru.c

Fix outdated comments in slru.c

SlruRecentlyUsed() is an inline function since 53c2a97a9266, not a
macro. The description of long_segment_names was missing at the top of
SimpleLruInit(), part forgotten in 4ed8f0913bfd.

Author: Julien Rouhaud <rjuju123@gmail.com>
Discussion: /messages/by-id/aLpBLMOYwEQkaleF@jrouhaud
Backpatch-through: 17

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fd3b972d78f60292b7aed8ea06f941a49d0b7144

Modified Files
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src/backend/access/transam/slru.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)