Rephrase references to "time qualification".

Now that the relevant code has, for other reasons, moved out of
tqual.[ch], it seems time to refer to visiblity rather than time
qualification.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

doc/src/sgml/indexam.sgml

@@ -720,7 +720,7 @@ amparallelrescan (IndexScanDesc scan);
    the TIDs of all the tuples it has been told about that match the
    <firstterm>scan keys</firstterm>.  The access method is <emphasis>not</emphasis> involved in
    actually fetching those tuples from the index's parent table, nor in
-   determining whether they pass the scan's time qualification test or other
+   determining whether they pass the scan's visibility test or other
    conditions.
   </para>
 

src/backend/access/heap/heapam.c

@@ -1700,7 +1700,7 @@ heap_fetch(Relation relation,
 	tuple->t_tableOid = RelationGetRelid(relation);
 
 	/*
-	 * check time qualification of tuple, then release lock
+	 * check tuple visibility, then release lock
 	 */
 	valid = HeapTupleSatisfiesVisibility(tuple, snapshot, buffer);
 
@@ -2020,8 +2020,8 @@ heap_get_latest_tid(Relation relation,
 		}
 
 		/*
-		 * Check time qualification of tuple; if visible, set it as the new
-		 * result candidate.
+		 * Check tuple visibility; if visible, set it as the new result
+		 * candidate.
 		 */
 		valid = HeapTupleSatisfiesVisibility(&tp, snapshot, buffer);
 		CheckForSerializableConflictOut(valid, relation, &tp, buffer, snapshot);

src/backend/access/heap/heapam_visibility.c

@@ -1453,8 +1453,8 @@ HeapTupleIsSurelyDead(HeapTuple htup, TransactionId OldestXmin)
  * It's easy to check just infomask bits if the locker is not a multi; but
  * otherwise we need to verify that the updating transaction has not aborted.
  *
- * This function is here because it follows the same time qualification rules
- * laid out at the top of this file.
+ * This function is here because it follows the same visibility rules laid out
+ * at the top of this file.
  */
 bool
 HeapTupleHeaderIsOnlyLocked(HeapTupleHeader tuple)

src/backend/utils/cache/inval.c

@@ -5,7 +5,7 @@
  *
  *	This is subtle stuff, so pay attention:
  *
- *	When a tuple is updated or deleted, our standard time qualification rules
+ *	When a tuple is updated or deleted, our standard visibility rules
  *	consider that it is *still valid* so long as we are in the same command,
  *	ie, until the next CommandCounterIncrement() or transaction commit.
  *	(See acces/heap/heapam_visibility.c, and note that system catalogs are