docs: transformer code needs more types and docstrings

[boneskull]

There's a lot of any types still hanging around in here. because it's not yet documented how a user can create a plugin, documentation of the Transformer API was not a priority.

especially concerning I think is the field definition bit. transformer A needs to be able to tell transformer B about its fields and how to display them, if applicable. The key is the "if applicable" part, which indicates to me this is a leaky abstraction. I'd like to rethink this, because it's really fussy to work with--especially when the fields can themselves be overridden by user configuration (YAGNI, perhaps).