Update database guidelines: how to combine "disable_ddl_transaction" steps with other steps in DB migrations

Update database guidelines:

• describe how combine multiple disable_ddl_transaction! migrations;
• clarify why disable_ddl_transaction! must not be combined with "regular" migration steps.

See: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9283/diffs#190bcafe00eea6127834bd45d95691e8f4e68886_0_15

Also related (needs finishing): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21177