Refine nbtree = redundancy preprocessing comment.

Spell out how a = key associated with a SAOP array renders a > key
against the same index column redundant at the relevant point inside
_bt_preprocess_keys.

Follow-up to commit 5bf748b.

Author: petergeoghegan

src/backend/access/nbtree/nbtutils.c

@@ -2668,8 +2668,14 @@ _bt_preprocess_keys(IndexScanDesc scan)
 
 			/*
 			 * If = has been specified, all other keys can be eliminated as
-			 * redundant.  If we have a case like key = 1 AND key > 2, we can
-			 * set qual_ok to false and abandon further processing.
+			 * redundant.  Note that this is no less true if the = key is
+			 * SEARCHARRAY; the only real difference is that the inequality
+			 * key _becomes_ redundant by making _bt_compare_scankey_args
+			 * eliminate the subset of elements that won't need to be matched.
+			 *
+			 * If we have a case like "key = 1 AND key > 2", we set qual_ok to
+			 * false and abandon further processing.  We'll do the same thing
+			 * given a case like "key IN (0, 1) AND key > 2".
 			 *
 			 * We also have to deal with the case of "key IS NULL", which is
 			 * unsatisfiable in combination with any other index condition. By