GUC table: Add description to computed variables

Per suggestion from Kyotaro Horiguchi
Discussion: https://postgr.es/m/20240229.130404.1411153273308142188.horikyota.ntt@gmail.com

src/backend/utils/misc/guc_tables.c

@@ -2287,7 +2287,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"commit_timestamp_buffers", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Sets the size of the dedicated buffer pool used for the commit timestamp cache."),
-			NULL,
+			gettext_noop("Specify 0 to have this value determined as a fraction of shared_buffers."),
 			GUC_UNIT_BLOCKS
 		},
 		&commit_timestamp_buffers,
@@ -2342,7 +2342,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"subtransaction_buffers", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Sets the size of the dedicated buffer pool used for the sub-transaction cache."),
-			NULL,
+			gettext_noop("Specify 0 to have this value determined as a fraction of shared_buffers."),
 			GUC_UNIT_BLOCKS
 		},
 		&subtransaction_buffers,
@@ -2353,7 +2353,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"transaction_buffers", PGC_POSTMASTER, RESOURCES_MEM,
 			gettext_noop("Sets the size of the dedicated buffer pool used for the transaction status cache."),
-			NULL,
+			gettext_noop("Specify 0 to have this value determined as a fraction of shared_buffers."),
 			GUC_UNIT_BLOCKS
 		},
 		&transaction_buffers,
@@ -2868,7 +2868,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"wal_buffers", PGC_POSTMASTER, WAL_SETTINGS,
 			gettext_noop("Sets the number of disk-page buffers in shared memory for WAL."),
-			NULL,
+			gettext_noop("Specify -1 to have this value determined as a fraction of shared_buffers."),
 			GUC_UNIT_XBLOCKS
 		},
 		&XLOGbuffers,