Update documentation wrt. using GRUB + Secure Boot for USB boot on EFI 64-bit

Originally created by @intrigeri on #17492 (Redmine)

don’t recommend disabling Secure Boot (i.e. delete 2 lines in install/inc/steps/restart_first_time.inline.mdwn) except on Apple computers (Apple’s implementation of Secure Boot only allows starting macOS and Windows)
adding/changing boot options
booting in “Troubleshooting mode”

Feature Branch: web/release-4.5

Edited May 15, 2020 by intrigeri

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