add syntax feature documentation (#1256)

Author: minhkhul

docs/api/covidcast.md

@@ -113,6 +113,16 @@ and lists.
 The current set of signals available for each data source is returned by the
 [`covidcast_meta`](covidcast_meta.md) endpoint.
 
+#### Alternate Required Parameters
+
+The following parameters help specify multiple source-signal, timetype-timevalue or geotype-geovalue pairs. Use them instead of the usual required parameters.
+
+| Parameter | Replaces | Format | Description | Example |
+| --- | --- | --- | --- | --- |
+| `signal` | `data_source`, `signal` | `signal={source}:{signal1},{signal2}` | Specify multiple source-signal pairs, grouped by source | `signal=src1:sig1`, `signal=src1:sig1,sig2`, `signal=src1:*`, `signal=src1:sig1;src2:sig3` |
+| `time` | `time_type`, `time_values` | `time={timetype}:{timevalue1},{timevalue2}` | Specify multiple timetype-timevalue pairs, grouped by timetype | `time=day:*`, `time=day:20201201`, `time=day:20201201,20201202`, `time=day:20201201-20201204` |
+| `geo` | `geo_type`, `geo_value` | `geo={geotype}:{geovalue1},{geovalue2}` | Specify multiple geotype-geovalue pairs, grouped by geotype | `geo=fips:*`, `geo=fips:04019`, `geo=fips:04019,19143`, `geo=fips:04019;msa:40660`, `geo=fips:*;msa:*` |
+
 #### Optional
 
 Estimates for a specific `time_value` and `geo_value` are sometimes updated
@@ -209,6 +219,12 @@ The `fields` parameter can be used to limit which fields are included in each re
 
 https://api.delphi.cmu.edu/epidata/covidcast/?data_source=fb-survey&signal=smoothed_cli&time_type=day&geo_type=county&time_values=20200406-20200410&geo_value=06001
 
+or
+
+https://api.delphi.cmu.edu/epidata/covidcast/?signal=fb-survey:smoothed_cli&time=day:20200406-20200410&geo=county:06001
+
+Both of these URLs are equivalent and can be used to get the following result:
+
 ```json
 {
   "result": 1,