ext4: Improve ext4 Kconfig test

Now that ext4 driver must be used to access ext3 filesystems, improve
the Kconfig help text to better explain that using ext4 driver to access
the filesystem is fully compatible with the old ext3 driver.

Acked-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jan Kara <jack@suse.com>

fs/ext4/Kconfig

@@ -49,12 +49,13 @@ config EXT4_FS
 	  up fsck time.  For more information, please see the web pages at
 	  http://ext4.wiki.kernel.org.
 
-	  The ext4 filesystem will support mounting an ext3
-	  filesystem; while there will be some performance gains from
-	  the delayed allocation and inode table readahead, the best
-	  performance gains will require enabling ext4 features in the
-	  filesystem, or formatting a new filesystem as an ext4
-	  filesystem initially.
+	  The ext4 filesystem supports mounting an ext3 filesystem; while there
+	  are some performance gains from the delayed allocation and inode
+	  table readahead, the best performance gains require enabling ext4
+	  features in the filesystem using tune2fs, or formatting a new
+	  filesystem as an ext4 filesystem initially. Without explicit enabling
+	  of ext4 features, the on disk filesystem format stays fully backward
+	  compatible.
 
 	  To compile this file system support as a module, choose M here. The
 	  module will be called ext4.