Document support for multiple geo_values

[krivard]

#221 got fixed, but I neglected to check for docs, so it's a secret feature until we update them.

Affected file: https://github.com/cmu-delphi/delphi-epidata/blob/main/docs/api/covidcast.md

[sgratzl]

also haven't seen the multiple signal support in the docs, thus I didn't included the multiple regions in the docs assuming it was also intentional