f2fs: update f2fs documentation for inline dir support

This patch adds descriptions for the inline dir support in f2fs document.
Signed-off-by: NChao Yu <chao2.yu@samsung.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -122,6 +122,10 @@ disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
 inline_xattr           Enable the inline xattrs feature.
 inline_data            Enable the inline data feature: New created small(<~3.4k)
                        files can be written into inode block.
+inline_dentry          Enable the inline dir feature: data in new created
+                       directory entries can be written into inode block. The
+                       space of inode block which is used to store inline
+                       dentries is limited to ~3.4k.
 flush_merge	       Merge concurrent cache_flush commands as much as possible
                        to eliminate redundant command issues. If the underlying
 		       device handles the cache_flush command relatively slowly,