man/kernel-install: fix formatting and document /etc/kernel/devicetree

man/kernel-install: fix formatting and document /etc/kernel/devicetree

Each filename should be a separate <term>, so that they separated in the
formatted text. Also, we list files in documentation in priority order, but
here they were in reverse order. Also, rework the description of
$KERNEL_INSTALL_CONF_ROOT to say that it makes kernel-install not look at the
other files. This requires some more words, so make this a separate paragraph
and refer from individual items to it. Also, drop some sentences with "Read by
...", they were already outdated.

Partial fix for #28771.

Co-authored-by: Emil Renner Berthing <systemd@esmil.dk>