btrfs-progs: docs: update a note related to orphan roots cleanup in Seeding

There is a note about a bug that mount a fs RO first, then remount it
RW, will make btrfs to skip the orphan roots cleanup.

However it's no longer the case after kernel commit 44c0ca211a4d
("btrfs: lift read-write mount setup from mount and remount"), as that
commit unify the pre-RW mount checks, and will always do the orphan
roots cleanup.

Just update the note so that it won't cause any confusion.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

Documentation/ch-seeding-device.rst

@@ -26,11 +26,13 @@ filesystem at :file:`/path` ready for use.
 
 .. note::
 
-        There is a known bug with using remount to make the mount writeable:
+        There was a known bug with using remount to make the mount writeable:
         remount will leave the filesystem in a state where it is unable to
         clean deleted snapshots, so it will leak space until it is unmounted
         and mounted properly.
 
+	That bug has fixed in 5.11 and newer kernels.
+
 Furthermore, deleting the seeding device from the filesystem can turn it into
 a normal filesystem, provided that the writable device can also contain all the
 data from the seeding device.