Document new syntax features for covidcast endpoint

[krivard]

We currently only disclose the backwards-compatible set of query parameters in the API documentation for covidcast:

• source
• rendered

However, several other useful ways of specifying which data to return are now supported, including:

• signal={src}:{sig1},{sig2} - specify multiple source-signal pairs, grouped by source
• time={type}:{val1},{val2} - specify multiple timetype-timevalue pairs, grouped by timetype
• geo={type}:{val1},{val2} - specify multiple geotype-geovalue pairs, grouped by geotype

We should:

• Examine the source code to figure out what new parameters are supported
• Add their correct usage to the API documentation