ext4: update ext4 documentation

Add documentation for mount options and ioctls to
Documentation/filesystem/ext4.txt, which has not been udpated for some
time.  Also add for ext4 sysfs tunables to the
Documentation/ABI/testing/sysfs-fs-ext4 file, and fix a few
typographical errors in that file.

https://bugzilla.kernel.org/show_bug.cgi?id=9423Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

Documentation/ABI/testing/sysfs-fs-ext4

@@ -48,7 +48,7 @@ Description:
 		 will have its blocks allocated out of its own unique
 		 preallocation pool.
 
-What:		/sys/fs/ext4/<disk>/inode_readahead
+What:		/sys/fs/ext4/<disk>/inode_readahead_blks
 Date:		March 2008
 Contact:	"Theodore Ts'o" <tytso@mit.edu>
 Description:
@@ -85,7 +85,14 @@ Date:		June 2008
 Contact:	"Theodore Ts'o" <tytso@mit.edu>
 Description:
 		Tuning parameter which (if non-zero) controls the goal
-		inode used by the inode allocator in p0reference to
-		all other allocation hueristics.  This is intended for
+		inode used by the inode allocator in preference to
+		all other allocation heuristics.  This is intended for
 		debugging use only, and should be 0 on production
 		systems.
+
+What:		/sys/fs/ext4/<disk>/max_writeback_mb_bump
+Date:		September 2009
+Contact:	"Theodore Ts'o" <tytso@mit.edu>
+Description:
+		The maximum number of megabytes the writeback code will
+		try to write out before move on to another inode.