filesystem: document _pre_shutdown and link to systemd-shutdown impl

We decided to revert the use of systemd-shutdown for exporting zpools
because it didn't address the issue we were trying to solve. However,
dropping scripts for systemd-shutdown is an interesting implementation
from an architecture standpoint, because it can be executed even if
Subiquity does not cleanly exit.

Let's document the _pre_shutdown function and link to the original
systemd-shutdown implementation that we came up with for exporting
zpools earlier.

Signed-off-by: Olivier Gayot <[email protected]>
(cherry picked from commit f36c828)

subiquity/server/controllers/filesystem.py

@@ -1633,6 +1633,19 @@ def make_autoinstall(self):
         return r
 
     async def _pre_shutdown(self):
+        """This function is executed just before rebooting and after copying
+        logs to the target. This means bugs reports are unlikely to include
+        execution logs from this function and therefore diagnosing issues is a
+        challenge. Let's try to keep it as simple as possible.
+
+        Another approach to execute commands before reboot is to place scripts in
+        /usr/lib/systemd/system-shutdown and lean on systemd-shutdown(8) to
+        execute them after unmounting most file-systems.
+
+        See this PR for an example (the PR was eventually reverted because it
+        didn't address the issue we tried to solve at the time).
+        https://github.com/canonical/subiquity/pull/2064
+        """
         if not self.reset_partition_only:
             # /target is mounted only if the installation was actually started.
             try: