improved docs/api-guide/filtering.md for better understanding

[journpy]

I found it difficult locating where to override the SEARCH_PARAM setting when reading the docs:

By default, the search parameter is named 'search', but this may be overridden with the SEARCH_PARAM setting.

In order to help other users who might have this challenge, I added a clause to show that this setting can be overridden in the REST_FRAMEWORK settings. Same with ORDERING_PARAM.

[browniebroke]

That's indeed implied, but explicit is better 👍