f2fs: fix to document reserved special compression extension

There is one reserved special compression extension: '*', which
could be set via 'compress_extension="*"' mount option to enable
compression for all files.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.rst

@@ -258,6 +258,8 @@ compress_extension=%s  Support adding specified extension, so that f2fs can enab
                        on compression extension list and enable compression on
                        these file by default rather than to enable it via ioctl.
                        For other files, we can still enable compression via ioctl.
+                       Note that, there is one reserved special extension '*', it
+                       can be set to enable compression for all files.
 ====================== ============================================================
 
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