Improve guidelines on filtering

[a-akimov]

The filtering guidelines could use some more details:

• What the filter syntax should be. For instance, the example in the style guide uses filtering via =, but the current implementation of the Partner API supports < <= > >= iexact isnull. Is the current implementation compliant (since it uses query params like the rule states) or not (since the actual filtering syntax differs from the style guide example)?

• How to design multi-value filters (e.g. "return invoices with the 'draft' or 'issued' status). Whether to use ?key=value1&key=value2 or ?key=value1,value2 or some other syntax.

• Consider also mentioning what the response should be if the filter returns nothing. I guess it should be 200 ОК {"data":[]}  rather than, say, 404 Not Found.