NCHS release: fix wording of flu signals

Author: krivard

docs/api/covidcast-signals/nchs-mortality.md

@@ -24,8 +24,8 @@ certificate data received and coded by the National Center for Health Statistics
 | `deaths_covid_incidence_prop` | Number of weekly new deaths with confirmed or presumed COVID-19, per 100,000 population |
 | `deaths_allcause_incidence_num` | Number of weekly new deaths from all causes |
 | `deaths_allcause_incidence_prop` | Number of weekly new deaths from all causes, per 100,000 population |
-| `deaths_flu_incidence_num` | Number of weekly new deaths involving Influenza |
-| `deaths_flu_incidence_prop` | Number of weekly new deaths involving Influenza, per 100,000 population |
+| `deaths_flu_incidence_num` | Number of weekly new deaths involving Influenza and at least one of (Pneumonia, COVID-19)|
+| `deaths_flu_incidence_prop` | Number of weekly new deaths involving Influenza and at least one of (Pneumonia, COVID-19), per 100,000 population |
 | `deaths_pneumonia_notflu_incidence_num` | Number of weekly new deaths involving Pneumonia, excluding Influenza deaths |
 | `deaths_pneumonia_notflu_incidence_prop` | Number of weekly new deaths involving Pneumonia, excluding Influenza deaths, per 100,000 population |
 | `deaths_covid_and_pneumonia_notflu_incidence_num`| Number of weekly new deaths involving COVID-19 and Pneumonia, excluding Influenza |