test: add comment about custom autorelabel.service

Salvaged from ead814a0b0.

test/units/autorelabel.service

@@ -1,4 +1,15 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
+
+# We use a custom autorelabel service instead of the SELinux provided set of
+# units & a generator, since the generator overrides the default target to the
+# SELinux one when it detects /.autorelabel. However, we use systemd.unit= on
+# the kernel command cmdline which always takes precedence, rendering all
+# SELinux efforts useless. Also, pulling in selinux-autorelabel.service
+# explicitly doesn't work either, as it doesn't check for the presence of
+# /.autorelabel and does the relabeling unconditionally which always ends with
+# a reboot, so we end up in a reboot loop (and it also spews quite a lot of
+# errors as it wants /etc/fstab and dracut-initramfs-restore).
+
 [Unit]
 Description=Relabel all filesystems
 DefaultDependencies=no