Add note about safety_assured not carrying over during rollback

[55728]

Fixes #284.

When using safety_assured with add_column inside a change method, the safety_assured context does not carry over when Rails reverts the migration. This can be confusing when check_down is enabled, as the rollback hits the remove_column check with no indication of why safety_assured didn't help.

Building on d15e3fa which improved the error output formatting for rollbacks, this adds a brief note to the message content itself suggesting the use of up and down methods instead.