Fix typos and duplicate words

This fixes various typos, duplicated words, and tiny bits of whitespace
mainly in code comments but also in docs.

Author: Daniel Gustafsson <daniel@yesql.se>
Author: Heikki Linnakangas <hlinnaka@iki.fi>
Author: Alexander Lakhin <exclusion@gmail.com>
Author: David Rowley <dgrowleyml@gmail.com>
Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/3F577953-A29E-4722-98AD-2DA9EFF2CBB8@yesql.se

contrib/amcheck/expected/check_btree.out

@@ -2,7 +2,7 @@ CREATE TABLE bttest_a(id int8);
 CREATE TABLE bttest_b(id int8);
 CREATE TABLE bttest_multi(id int8, data int8);
 CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
--- Stabalize tests
+-- Stabilize tests
 ALTER TABLE bttest_a SET (autovacuum_enabled = false);
 ALTER TABLE bttest_b SET (autovacuum_enabled = false);
 ALTER TABLE bttest_multi SET (autovacuum_enabled = false);

contrib/amcheck/sql/check_btree.sql

@@ -3,7 +3,7 @@ CREATE TABLE bttest_b(id int8);
 CREATE TABLE bttest_multi(id int8, data int8);
 CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
 
--- Stabalize tests
+-- Stabilize tests
 ALTER TABLE bttest_a SET (autovacuum_enabled = false);
 ALTER TABLE bttest_b SET (autovacuum_enabled = false);
 ALTER TABLE bttest_multi SET (autovacuum_enabled = false);

contrib/amcheck/verify_nbtree.c

@@ -3036,7 +3036,7 @@ bt_normalize_tuple(BtreeCheckState *state, IndexTuple itup)
 	 * In the heap, tuples may contain short varlena datums with both 1B
 	 * header and 4B headers.  But the corresponding index tuple should always
 	 * have such varlena's with 1B headers.  So, if there is a short varlena
-	 * with 4B header, we need to convert it for for fingerprinting.
+	 * with 4B header, we need to convert it for fingerprinting.
 	 *
 	 * Note that we rely on deterministic index_form_tuple() TOAST compression
 	 * of normalized input.