sql: fix ALTER DATABASE help text

Prior to this commit, the CLI help text shown for ALTER DATABASE
was incomplete and incorrect. This commit fixes the help text to
include some omitted options and fixes ADD REGIONS and DROP REGIONS
to say ADD REGION and DROP REGION.

Release note (cli change): Fixed the CLI help text for ALTER DATABASE
to show correct options for ADD REGION and DROP REGION, and include
some missing options such as CONFIGURE ZONE.

pkg/sql/parser/sql.y

@@ -1483,10 +1483,11 @@ alter_sequence_options_stmt:
 // %Category: DDL
 // %Text:
 // ALTER DATABASE <name> RENAME TO <newname>
+// ALTER DATABASE <name> CONFIGURE ZONE <zone config>
 // ALTER DATABASE <name> OWNER TO <newowner>
 // ALTER DATABASE <name> CONVERT TO SCHEMA WITH PARENT <name>
-// ALTER DATABASE <name> ADD REGIONS <regions>
-// ALTER DATABASE <name> DROP REGIONS <regions>
+// ALTER DATABASE <name> ADD REGION [IF NOT EXISTS] <region>
+// ALTER DATABASE <name> DROP REGION [IF EXISTS] <region>
 // ALTER DATABASE <name> PRIMARY REGION <region>
 // ALTER DATABASE <name> SURVIVE <failure type>
 // %SeeAlso: WEBDOCS/alter-database.html