Fix misc typos, mostly in comments.

A collection of typos I happened to spot while reading code, as well as
grepping for common mistakes.

Backpatch to all supported versions, as applicable, to avoid conflicts
when backporting other commits in the future.

src/test/regress/expected/rowsecurity.out

@@ -2108,7 +2108,7 @@ SET SESSION AUTHORIZATION regress_rls_bob;
 INSERT INTO document VALUES (79, (SELECT cid from category WHERE cname = 'technology'), 1, 'regress_rls_bob', 'technology book, can only insert')
     ON CONFLICT (did) DO UPDATE SET dtitle = EXCLUDED.dtitle RETURNING *;
 ERROR:  new row violates row-level security policy for table "document"
--- UPDATE path is taken here.  Existing tuple passes, since it's cid
+-- UPDATE path is taken here.  Existing tuple passes, since its cid
 -- corresponds to "novel", but default USING qual is enforced against
 -- post-UPDATE tuple too (as always when updating with a policy that lacks an
 -- explicit WCO), and so this fails:

src/test/regress/expected/rules.out

@@ -2818,7 +2818,7 @@ Rules:
  NOTIFY rules_src_deletion
 
 --
--- Ensure a aliased target relation for insert is correctly deparsed.
+-- Ensure an aliased target relation for insert is correctly deparsed.
 --
 create rule r4 as on insert to rules_src do instead insert into rules_log AS trgt SELECT NEW.* RETURNING trgt.f1, trgt.f2;
 create rule r5 as on update to rules_src do instead UPDATE rules_log AS trgt SET tag = 'updated' WHERE trgt.f1 = new.f1;

src/test/regress/sql/rowsecurity.sql

@@ -781,7 +781,7 @@ SET SESSION AUTHORIZATION regress_rls_bob;
 INSERT INTO document VALUES (79, (SELECT cid from category WHERE cname = 'technology'), 1, 'regress_rls_bob', 'technology book, can only insert')
     ON CONFLICT (did) DO UPDATE SET dtitle = EXCLUDED.dtitle RETURNING *;
 
--- UPDATE path is taken here.  Existing tuple passes, since it's cid
+-- UPDATE path is taken here.  Existing tuple passes, since its cid
 -- corresponds to "novel", but default USING qual is enforced against
 -- post-UPDATE tuple too (as always when updating with a policy that lacks an
 -- explicit WCO), and so this fails:

src/test/regress/sql/rules.sql

@@ -1007,7 +1007,7 @@ create rule r3 as on delete to rules_src do notify rules_src_deletion;
 \d+ rules_src
 
 --
--- Ensure a aliased target relation for insert is correctly deparsed.
+-- Ensure an aliased target relation for insert is correctly deparsed.
 --
 create rule r4 as on insert to rules_src do instead insert into rules_log AS trgt SELECT NEW.* RETURNING trgt.f1, trgt.f2;
 create rule r5 as on update to rules_src do instead UPDATE rules_log AS trgt SET tag = 'updated' WHERE trgt.f1 = new.f1;