Document InferDict and Message typed dicts

[fritzo]

Resolves #3322

This adds key descriptions to InferDict and Message. Also adds the enumerate key to InferDict, which is the most commonly used key in our documentation. (I guess that would have been added once we add types to pyro.infer.*.)

There are a few ways to document a TypedDict. I've opted to add a sphinx definition list in the main docstring.

• pro: docs show up in both sphinx (docs.pyro.ai) and vscode hover
• con: docs are somewhat duplicated in sphinx
• con: key descriptions are separate from declaration in code

The second best alternative would be to add a """docstring""" below each key declaration, but those wouldn't show up in vscode hover, which I believe is more important.