Please document purpose of the pickle extension in more detail

[flyser]

Can you please document the purpose of the pickling_support module in more detail? It seems like I can pickle and unpickle exceptions out of the box, so what do I gain from using this module?

[ionelmc]

Pickled exception chains, see https://github.com/ionelmc/python-tblib/pull/54/files

Not sure where to document this, please make a suggestion.