btrfs-progs: docs: update chattr attribute for NOCOMPRESS

There were plans to add X as flag to set/unset the btrfs NOCOMPRESS
attribute but that never materialized. In e2fsprogs the letter 'm' has
been assigned to the same functionality and released in version 1.46.2.

Update the docs and mention that the compression options are
conflicting.

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

Documentation/btrfs-man5.asciidoc

@@ -846,6 +846,7 @@ Please note that compression is also affected by the mount options or the parent
 directory attributes.
 +
 When set on a directory, all newly created files will inherit this attribute.
+This attribute cannot be set with 'm' at the same time.
 
 *C*::
 'no copy-on-write', file data modifications are done in-place
@@ -867,16 +868,18 @@ and 'O_DSYNC'
 'immutable', no file data and metadata changes allowed even to the root user as
 long as this attribute is set (obviously the exception is unsetting the attribute)
 
+*m*::
+'no compression', permanently turn off compression on the given file. Any
+compression mount options will not affect this file. (`chattr` support added in
+1.46.2)
++
+When set on a directory, all newly created files will inherit this attribute.
+This attribute cannot be set with 'c' at the same time.
+
 *S*::
 'synchronous updates', for more details search `open`(2) for 'O_SYNC' and
 'O_DSYNC'
 
-*X*::
-'no compression', permanently turn off compression on the given file. Any
-compression mount options will not affect this file.
-+
-When set on a directory, all newly created files will inherit this attribute.
-
 No other attributes are supported.  For the complete list please refer to the
 `chattr`(1) manual page.