Update PG_CACHE_LINE_SIZE description.

PG_CACHE_LINE_SIZE was originally only used in xlog.c, but this hasn't
been true for a very long time and is now wildly used, so modify its
description to not mention any explicit source code file.

Author: Julien Rouhaud
Discussion: https://www.postgresql.org/message-id/20230701074936.p3qcssl4t7murt2q@jrouhaud

src/include/pg_config_manual.h

@@ -217,13 +217,13 @@
 #define DEFAULT_EVENT_SOURCE  "PostgreSQL"
 
 /*
- * Assumed cache line size. This doesn't affect correctness, but can be used
- * for low-level optimizations. Currently, this is used to pad some data
- * structures in xlog.c, to ensure that highly-contended fields are on
- * different cache lines. Too small a value can hurt performance due to false
- * sharing, while the only downside of too large a value is a few bytes of
- * wasted memory. The default is 128, which should be large enough for all
- * supported platforms.
+ * Assumed cache line size.  This doesn't affect correctness, but can be used
+ * for low-level optimizations.  This is mostly used to pad various data
+ * structures, to ensure that highly-contended fields are on different cache
+ * lines.  Too small a value can hurt performance due to false sharing, while
+ * the only downside of too large a value is a few bytes of wasted memory.
+ * The default is 128, which should be large enough for all supported
+ * platforms.
  */
 #define PG_CACHE_LINE_SIZE		128