boot: move/adjust comments

boot: move/adjust comments

The comment in linux_exec() was based on Lennart's comment in
https://github.com/systemd/systemd/pull/37372#discussion_r2142340582, but
shortened. The original wording is more direct and at least for me easier to
grok, so adjust the comment to be more verbose again.

Also, move the comment from shim_loader_available() to the place where it used.
This function is for checking if the new thing is available, no need to describe
the old thing there.