Missing OEM bootloader unlocking in "bullhead" install instructions
As stated on the respective LineageOS wiki page:
Enable OEM unlock in the Developer options under device Settings, if present
This is necessary, otherwise it'll say
$ fastboot flashing unlock ... FAILED (remote: oem unlock is not allowed) finished. total time: 0.014s
In addition to that, since this is supposed to be easy for even not-so-techy users, I'd recommend adding instructions on how to get to the developer options in the first place, such as described on XDA Developers:
Go into Settings/About Phone, scroll down and click on “build number” continuously until you see a toast notification telling you that you've enabled Developer Options. Go back to your Settings menu and enter Developer Options, scroll down and click on the “Enable OEM Unlock” checkbox, also make sure you enable USB Debugging while you're in the Developer Options menu.
...otherwise even executing the
adb reboot bootloader step won't work.