docs: add lsblk --merge to TODO

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

Documentation/TODO

@@ -34,12 +34,6 @@ lscpu
 
 lsblk
 -----
-  - re-write the way how lsblk internally holds info about devices. Now we
-    compose only output (by libsmartcols). It would be better to have in memory
-    complete tree of the devices and generate the output from this tree. It will
-    make code more readable and allow to generate output in more ways -- for
-    example for RAIDs (https://github.com/karelzak/util-linux/issues/616)
-
   - currently it does not show mountpoint for all devices in btrfs RAID. It's because
     /proc/#/mountinfo contains reference to the one device only. Maybe we can add some
     btrfs specific code to provide a better output for FS based stacks. Not sure.
@@ -49,6 +43,22 @@ lsblk
     and it may be confusing for end-users when FS to DEV mapping is not 1:1 (RAIDs, btrfs, 
     ...)
 
+  - add --merge option to merge the same trees
+	sdb       8:16   0   204M  0 disk  
+	 └─md127   9:127  0 611.5M  0 raid0 
+	sdc       8:32   0   204M  0 disk  
+	 └─md127   9:127  0 611.5M  0 raid0 
+	sdd       8:48   0   204M  0 disk  
+	 └─md127   9:127  0 611.5M  0 raid0 
+    to
+	sdb|       8:16   0   204M  0 disk  
+	sdc|       8:32   0   204M  0 disk  
+	sdd|       8:48   0   204M  0 disk  
+	   └─md127   9:127  0 611.5M  0 raid0 
+
+    This feature will require change to libsmartcols to support "multi-parent"
+    output formatting (more than one parent).
+
 nsenter(1)
 ----------
  - read the default UID and GID from the target process.