f2fs: document: add description about compressed space handling

User or developer may still be confused about why f2fs doesn't expose
compressed space to userspace, add description about compressed space
handling policy into f2fs documentation.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.rst

@@ -819,6 +819,14 @@ Compression implementation
   * chattr +c file
   * chattr +c dir; touch dir/file
   * mount w/ -o compress_extension=ext; touch file.ext
+  * mount w/ -o compress_extension=*; touch any_file
+
+- At this point, compression feature doesn't expose compressed space to user
+  directly in order to guarantee potential data updates later to the space.
+  Instead, the main goal is to reduce data writes to flash disk as much as
+  possible, resulting in extending disk life time as well as relaxing IO
+  congestion. Alternatively, we've added ioctl interface to reclaim compressed
+  space and show it to user after putting the immutable bit.
 
 Compress metadata layout::