Add note about default behavior of server backup
Add note about default behavior of server backup

Both mysqldump and mysqlbackup providers by default back up all databases in the database (i.e. when an empty array is specfied).
This patch documents that behavior.
See templates/xtrabackup.sh.erb and templates/mysqlbackup.sh.erb
simp/puppetlabs-mysql,DavidS/puppetlabs-mysql,puppetlabs/puppetlabs-mysql,puppetlabs/puppetlabs-mysql,simp/puppetlabs-mysql,markasammut/puppetlabs-mysql,markasammut/puppetlabs-mysql,DavidS/puppetlabs-mysql