Document multiparameter patern.

[carltongibson]

You need to filter on more that one parameter? Use a multi-widget to have all values passes as one to a single filter.

• Document this with an example.
• Does it merit a helper class?

c.f. #977 (comment)

[rpkilby]

Note that this isn't fully compatible with API use. e.g., take the range* multi filters. The actual query params have after and before suffixes, however validation errors are raised under the unsuffixed filter name. There is no way to distinguish which sub-param the error applied to.

The multi-widget incompatibility should also be documented in the DRF integration section.

[carltongibson]

Filtering is done as per the Spanning multi-valued relationships Django docs. If you need to filter two parameters in a single call to filter(), you need to be creating a (multi-)filter that takes the (e.g. two) parameters and applies them in a single step.

There's an example for this pattern here: https://gitlab.com/-/snippets/2237049

[carltongibson]

Different example in #1672 (comment)