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: https://postgr.es/m/
20240626002747.dc.nmisch@google.com
Backpatch-through: 17