Clarify migrate:rollback command description

[D1360-64RC14]

Just +1 developer that felt into that "Rollback the last database migration" that, actually, rolls back the previous batch; not the migration.

Reading the command description it's easy to assume it will do a migrate:rollback --step=1.

Maybe also add a confirmation for these destructive operations, like showing the amount of migrations or which of them will be rolled back.

I know there's a migrate:rollback --pretend, but no one will think of it in a local development environment.