ext4: Update documentation about quota mount options

Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

Documentation/filesystems/ext4.txt

@@ -282,9 +282,16 @@ stripe=n		Number of filesystem blocks that mballoc will try
 			to use for allocation size and alignment. For RAID5/6
 			systems this should be the number of data
 			disks *  RAID chunk size in file system blocks.
-delalloc	(*)	Deferring block allocation until write-out time.
-nodelalloc		Disable delayed allocation. Blocks are allocation
-			when data is copied from user to page cache.
+
+delalloc	(*)	Defer block allocation until just before ext4
+			writes out the block(s) in question.  This
+			allows ext4 to better allocation decisions
+			more efficiently.
+nodelalloc		Disable delayed allocation.  Blocks are allocated
+			when the data is copied from userspace to the
+			page cache, either via the write(2) system call
+			or when an mmap'ed page which was previously
+			unallocated is written for the first time.
 
 max_batch_time=usec	Maximum amount of time ext4 should wait for
 			additional filesystem operations to be batch