btrfs-progs: docs: fix incorrect description about compression with O_DIRECT

It was reported in https://lore.kernel.org/linux-btrfs/e7ce9995-93cb-4904-875c-684d4494765f@web.de/
that compression does not happen on direct io files. This is incorrectly
documented that it works but this is not true. Compression works on
buffered writes and relies on page cache, while direct io avoids that
and takes a different path in code.

[ci skip]

Pull-request: #764
Author: HAN Yuwei <hrx@bupt.moe>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: hanyuwei70

Documentation/ch-compression.rst

@@ -145,9 +145,9 @@ Compatibility
 -------------
 
 Compression is done using the COW mechanism so it's incompatible with
-*nodatacow*. Direct IO works on compressed files but will fall back to buffered
-writes and leads to recompression. Currently *nodatasum* and compression don't
-work together.
+*nodatacow*. Direct IO read works on compressed files but will fall back to
+buffered writes and leads to no compression even if force compression is set.
+Currently *nodatasum* and compression don't work together.
 
 The compression algorithms have been added over time so the version
 compatibility should be also considered, together with other tools that may