I#22893 - Update mysqldump command on backups docs

When you set the option "--no-data" on the first command, then it should
*not* try and do a LOCK on the "sessions" table cause obvious you're not
going to get data. This bug was fixed only in mysql-server 5.6, so lets
just suggest adding "--single-transaction" no matter the version for
simplicity's sake.
Merge branch '4.0/backups-mysql-patch' into 4.0-trunk