Improve some GUC description strings

It is not our usual style to use "we" in messages.  Also, remove some
noise words.  Backpatch to 15.

Noted by Kyotaro Horiguchi.

Discussion: https://postgr.es/m/20220914.111507.13049297635620898.horikyota.ntt@gmail.com

src/backend/utils/misc/guc_tables.c

@@ -2591,7 +2591,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"wal_decode_buffer_size", PGC_POSTMASTER, WAL_RECOVERY,
 			gettext_noop("Buffer size for reading ahead in the WAL during recovery."),
-			gettext_noop("This controls the maximum distance we can read ahead in the WAL to prefetch referenced data blocks."),
+			gettext_noop("Maximum distance to read ahead in the WAL to prefetch referenced data blocks."),
 			GUC_UNIT_BYTE
 		},
 		&wal_decode_buffer_size,
@@ -3248,7 +3248,7 @@ struct config_int ConfigureNamesInt[] =
 	{
 		{"tcp_keepalives_count", PGC_USERSET, CONN_AUTH_TCP,
 			gettext_noop("Maximum number of TCP keepalive retransmits."),
-			gettext_noop("This controls the number of consecutive keepalive retransmits that can be "
+			gettext_noop("Number of consecutive keepalive retransmits that can be "
 						 "lost before a connection is considered dead. A value of 0 uses the "
 						 "system default."),
 		},