btrfs: add documentation around snapshot delete

Snapshot delete has some complicated looking code that is weirdly subtle
at times.  I've cleaned it up the best I can without re-writing it, but
there are still a lot of details that are non-obvious.  Add a bunch of
comments to the main parts of the code to help future developers better
understand the mechanics of snapshot deletion.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>