pgsql: Improve comments in slru.{c,h} about segment name format

Improve comments in slru.{c,h} about segment name format

slru.h described incorrectly how SLRU segment names are formatted
depending on the segment number and if long or short segment names are
used. This commit closes the gap with a better description, fitting
with the reality.

Reported-by: Noah Misch
Author: Aleksander Alekseev
Discussion: /messages/by-id/20240626002747.dc.nmisch@google.com
Backpatch-through: 17

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ffb0603929617f39d449e942abe96cdba36e7545

Modified Files
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src/backend/access/transam/slru.c | 15 +++++++++++++++
src/include/access/slru.h | 7 +++----
2 files changed, 18 insertions(+), 4 deletions(-)