btrfs-progs: docs: add a note about RW degraded mount

When a new chunk is allocated during a RW degraded mount, btrfs can
allocate new chunks with other profiles, like SINGLE, if the current
devices can no longer support the original profile.

This can cause unexpected chunk profiles, and the end user is
recommended to balance them out if they want to maintain the current
allocation profile.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: adam900710

Documentation/ch-mount-options.rst

@@ -180,6 +180,15 @@ degraded
         then the constraint of single/data is not satisfied and the filesystem
         cannot be mounted.
 
+	.. note::
+                During a degraded mount, if a new chunk is going to be
+                allocated and the existing devices cannot meet the requirements
+                for the current profile then mount will fall back to other profiles
+                like SINGLE.
+
+		It is recommended to check and balance chunks with unwanted profiles,
+		after a read-write degraded mount.
+
 .. duplabel:: mount-option-device
 
 device=<devicepath>