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/ref-cache.h

@@ -19,8 +19,11 @@
 #define __REFCACHE__
 
 struct btrfs_extent_info {
+	/* bytenr and num_bytes find the extent in the extent allocation tree */
 	u64 bytenr;
 	u64 num_bytes;
+
+	/* objectid and offset find the back reference for the file */
 	u64 objectid;
 	u64 offset;
 };