libmount: add comment, remove TODO item

Signed-off-by: Karel Zak <kzak@redhat.com>

Documentation/TODO

@@ -1,10 +1,6 @@
 
  Note that items with (!) have high priority.
 
-canonicalize
-------------
-  - reimplement realpath(3) (for lib/canonicalize()) to use fstatat(AT_NO_AUTOMOUNT)
-
 lsblk
 -----
   - currently it does not show mountpoint for all devices in btrfs RAID. It's because

libmount/src/context_umount.c

@@ -162,6 +162,10 @@ err:
 
 /* Check if there is something important in the utab file. The parsed utab is
  * stored in context->utab and deallocated by mnt_free_context().
+ *
+ * This function exists to avoid (if possible) /proc/self/mountinfo usage, so
+ * don't use thigs like mnt_resolve_target(), mnt_context_get_mtab() etc here.
+ * See lookup_umount_fs() for more details.
  */
 static int has_utab_entry(struct libmnt_context *cxt, const char *target)
 {