f2fs: update f2fs document

This patch describes the inline_xattr support in f2fs document.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Documentation/filesystems/f2fs.txt

@@ -119,6 +119,7 @@ active_logs=%u         Support configuring the number of active logs. In the
                        Default number is 6.
 disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
                        does not aware of cold files such as media files.
+inline_xattr           Enable the inline xattrs feature.
 
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 DEBUGFS ENTRIES