Clarify the security implications of 'failsafe' startup option

The documentation reads “The failsafe mode disables some features of the kernel and might work better on some computers. You can try this option if you think you are experiencing errors related to hardware compatibility while starting Tails.”[1]

Many users are able to boot and use Tails with this option, the documentation should clarify which “features of the kernel” are disabled, especially wrt. security features

[1] https://tails.boum.org/doc/first_steps/startup_options

Feature Branch: feature/11975-boot-menu-wording

Parent Task: #10178 (closed)

Original created by @elouann on 11365 (Redmine)