code: replace 'master' with 'primary' where appropriate.

Also changed "in the primary" to "on the primary", and added a few
"the" before "primary".

Author: Andres Freund
Reviewed-By: David Steele
Discussion: https://postgr.es/m/20200615182235.x7lch5n6kcjq4aue@alap3.anarazel.de

src/backend/access/nbtree/README

@@ -574,7 +574,7 @@ writers that insert on to the page being deleted.)
 
 During recovery all index scans start with ignore_killed_tuples = false
 and we never set kill_prior_tuple. We do this because the oldest xmin
-on the standby server can be older than the oldest xmin on the master
+on the standby server can be older than the oldest xmin on the primary
 server, which means tuples can be marked LP_DEAD even when they are
 still visible on the standby. We don't WAL log tuple LP_DEAD bits, but
 they can still appear in the standby because of full page writes. So

src/backend/access/nbtree/nbtxlog.c

@@ -932,7 +932,7 @@ btree_xlog_reuse_page(XLogReaderState *record)
 	 * RecentGlobalXmin test in _bt_page_recyclable() conceptually mirrors the
 	 * pgxact->xmin > limitXmin test in GetConflictingVirtualXIDs().
 	 * Consequently, one XID value achieves the same exclusion effect on
-	 * master and standby.
+	 * primary and standby.
 	 */
 	if (InHotStandby)
 	{