Improve description of DEFAULT_XLOG_SEG_SIZE in pg_config.h

This was incorrectly referring to --walsegsize, and its description is
rewritten in a clearer way.

Author: Ian Barwick, Tom Lane
Reviewed-by: Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/08534fc6-119a-c498-254e-d5acc4e6bf85@2ndquadrant.com

src/include/pg_config_manual.h

@@ -14,8 +14,8 @@
  */
 
 /*
- * This is default value for wal_segment_size to be used at initdb when run
- * without --walsegsize option. Must be a valid segment size.
+ * This is the default value for wal_segment_size to be used when initdb is run
+ * without the --wal-segsize option.  It must be a valid segment size.
  */
 #define DEFAULT_XLOG_SEG_SIZE	(16*1024*1024)