doc: document stale pidfile cleanup and new restart log

* src/pid.c: note the stale-pidfile-cleanup exception to the
  documented "Finit does not touch pid:! pidfiles" rule.
* doc/config/services.md: add a user-facing paragraph on the same.
* doc/ChangeLog.md: add Unreleased section covering this PR --
  stale pidfile cleanup, restart log with signal name and core
  dump flag, and the SIGUNKOWN typo fix.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/config/services.md

@@ -31,6 +31,14 @@ basename of the binary to guess the PID file to watch for the PID:
 `/var/run/ntpd.pid`.  If Finit guesses wrong, you have to submit the
 full `pid:!/path/to/file.pid`.
 
+With `pid:!/path`, the file belongs to the service: Finit reads it
+but does not create or remove it.  The one exception is *stale*
+cleanup — if the service dies without removing its own pidfile
+(SIGKILL, OOM, segfault), and the file still names the just-reaped
+PID, Finit removes it before the next retry.  This prevents daemons
+that refuse to start on an existing pidfile (e.g. `dbus-daemon`)
+from getting stuck in a crash-restart loop.
+
 **Example:**
 
 In the case of `ospfd` (below), we omit the `-d` flag (daemonize) to