Document DV holiday effects

Author: rumackaaron

docs/api/covidcast-signals/doctor-visits.md

@@ -51,6 +51,11 @@ States.
 
 Standard errors are not available for this data source.
 
+Due to changes in medical-seeking behavior on holidays, this data source has
+upward spikes in CLI around major holidays (i.e. Memorial Day, July 4, Labor
+Day, etc.). These spikes are not necessarily indicative of a true increase of
+COVID-like illness in a location.
+
 ## Qualifying Conditions
 
 We receive data on the following five categories of counts: