Document various version environment variables

At https://github.com/freedomofpress/kernel-builder/pull/34#pullrequestreview-1529288306 @rocodes wrote:
> Slight nit is it could be good to have a bit more documentation for our future selves (specifically, that setting `LINUX_MAJOR_VERSION` overrides `LINUX_VERSION`, were both to be set in `build-kernel.sh`), but not worth a hold-up. Thank you for these changes!