bundle2: move the 'close' method off the unpackermixin This method is unrelated to unpacking and only used by the unbundle20 class. We move it there for clarity.

bundle2: clarify the docstring of unpackermixin methods The unpackermixin is a utility used to implement the bundle2 protocol. It should not be used when writing part handlers. We update the docstring to clarify this.

bundle2: add documention to 'part.addparams' There are some non-obvious limitations on the parameters of this method. Add some documentation where people will likely look to understand how to use this API.