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

src/backend/access/transam/slru.c

@@ -72,6 +72,21 @@
 #include "storage/shmem.h"
 #include "utils/guc_hooks.h"
 
+/*
+ * Converts segment number to the filename of the segment.
+ *
+ * "path" should point to a buffer at least MAXPGPATH characters long.
+ *
+ * If ctl->long_segment_names is true, segno can be in the range [0, 2^60-1].
+ * The resulting file name is made of 15 characters, e.g. dir/123456789ABCDEF.
+ *
+ * If ctl->long_segment_names is false, segno can be in the range [0, 2^24-1].
+ * The resulting file name is made of 4 to 6 characters, as of:
+ *
+ *  dir/1234   for [0, 2^16-1]
+ *  dir/12345  for [2^16, 2^20-1]
+ *  dir/123456 for [2^20, 2^24-1]
+ */
 static inline int
 SlruFileName(SlruCtl ctl, char *path, int64 segno)
 {

src/include/access/slru.h

@@ -134,10 +134,9 @@ typedef struct SlruCtlData
 	bits16		bank_mask;
 
 	/*
-	 * If true, use long segment filenames formed from lower 48 bits of the
-	 * segment number, e.g. pg_xact/000000001234. Otherwise, use short
-	 * filenames formed from lower 16 bits of the segment number e.g.
-	 * pg_xact/1234.
+	 * If true, use long segment file names.  Otherwise, use short file names.
+	 *
+	 * For details about the file name format, see SlruFileName().
 	 */
 	bool		long_segment_names;