Standardize fields and their docstrings

[MartinKl]

Fields of structs that perform imports, graph_ops, or exports should always have a default value. When a default is not possible, such as for path attributes, it should be wrapped in an Option to opt for None as default. This might shift semantics, but is for the greater good.

Docstrings should always provide an example of how to use it in a toml file to provide a hint to the type.