Uniform docstrings

[MartinKl]

We need a standard way of documenting struct fields in the code, as the documentation is derived from this. We can use this issue to collect several problems and desiderata.

As an example, how do we want to make transparent to the user, whether or not an attribute (struct field) is obligatory. In most cases we mention the default value in the docstring, but to conclude from a non-mentioning of a default that a field is required seems a bit much.

[MartinKl]

Also, it would be nice to add a way of print a code snippet from the documentation in a way that it can be copied from command line, which is currently not possible with the terminal tables.

[MartinKl]

If we can modify the following function:
https://github.com/cyqsimon/documented/blob/e2047670133b5469c96650ee56f153b1f1259165/documented-macros/src/derive_impl.rs#L110

... in a way, that it also reads the field attributes and creates constants for optional and obligatory fields (i. e. which field carries serde(default... and which not), we could forward that information to the user.