docs: add #1266 to TODO file

References: https://github.com/karelzak/util-linux/issues/1266
Signed-off-by: Karel Zak <kzak@redhat.com>

Documentation/TODO

@@ -16,6 +16,13 @@ tests
   - add Coverity to GitHub Actions (see for example .github/workflows/coverity.yml
     in systemd repo).
 
+fstrim
+------
+  - "fstrim --fstab" uses root= from /proc/cmdline to get root FS when it's missing
+    in fstab file. This is fragile (due to missing root= or the root FS is not 
+    accessible). The best seems to parse mountinfo and use mnt_table_get_root_fs()
+    as a fallback solution.  https://github.com/karelzak/util-linux/issues/1266.
+
 script (lib/pty-session.c)
 --------------------------
   - (!) add #ifdefs and optional code for non-signalfd() systems