document the use of the f10 key to boot in the "adding boot options" documentation
there is a firmware limitation that prevent the ctrl+x key combination to boot linux from grub2 on macbooks (see https://bugzilla.redhat.com/show_bug.cgi?id=1497075 or https://bugzilla.redhat.com/show_bug.cgi?id=1253637), pressing f10 instead should work better, except on macbooks with touchbar, where pressing the fn key before f10 seems to be needed (https://support.apple.com/en-us/HT207240).
we should document f10 instead of Ctrl+X in https://tails.boum.org/doc/advanced_topics/boot_options/#grub
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