Btrfs: add and improve comments

This improves the comments at the top of many functions.  It didn't
dive into the guts of functions because I was trying to
avoid merging problems with the new allocator and back reference work.

extent-tree.c and volumes.c were both skipped, and there is definitely
more work todo in cleaning and commenting the code.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.h

@@ -27,7 +27,6 @@
 #include <linux/backing-dev.h>
 #include <linux/wait.h>
 #include <asm/kmap_types.h>
-#include "bit-radix.h"
 #include "extent_io.h"
 #include "extent_map.h"
 #include "async-thread.h"