Walker documentation partly outdated

[Technologicat]

The documentation and docstrings of Walker need updating:

• ctx is passed by name, not positionally.
• ctx is just an arbitrary user-defined name; it has no special meaning. This is essentially a mechanism to pass in arbitrary named arguments that will be propagated following the rules explained in the documentation.
• Some up-to-date usage examples with one and two ctx items would make the documentation easier to understand.

I could take a look at this and perhaps submit a PR later.

[Technologicat]

A cut at fixing this with minimal changes to the documentation is in Technologicat@6abbdf8