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>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 8399d6d..2e60398 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -519,6 +519,9 @@
 	.fs_flags	= FS_REQUIRES_DEV,
 };
 
+/*
+ * used by btrfsctl to scan devices when no FS is mounted
+ */
 static long btrfs_control_ioctl(struct file *file, unsigned int cmd,
 				unsigned long arg)
 {