Better documentation for pricing strategies

[tsmbland]

We currently have quite good descriptions of some of the pricing methods in docstrings in prices.rs (after #1021), however this information isn't easily accessible to users.

I'm not entirely sure where we'd put this in the documentation. I think we'd want a dedicated page for this, rather than putting it in the input file schema. Probably somewhere in the Model Description section (which is already very out of date anyway)