Argument Clinic: add reference for the DSL

[Erlend E. Aasland]

We need a better reference for the Argument Clinic DSL:

• legal syntax for the "function name" line
• legal syntax for parameter lines
• legal syntax for docstrings

[Lincoln]

Hey I would love to work on this, thanks.

[Erlend E. Aasland]

Hey I would love to work on this, thanks.

Sure, feel free to propose a PR!

[Lincoln]

Thanks,