Documentation update for smoothed safegraph signals

docs/api/covidcast-signals/safegraph.md

@@ -33,6 +33,10 @@ documentation](https://docs.safegraph.com/docs/social-distancing-metrics).
 | `full_time_work_prop` | The fraction of mobile devices that spent more than 6 hours at a location other than their home during the daytime (SafeGraph's `full_time_work_behavior_devices / device_count`) |
 | `part_time_work_prop` | The fraction of devices that spent between 3 and 6 hours at a location other than their home during the daytime (SafeGraph's `part_time_work_behavior_devices / device_count`) |
 | `median_home_dwell_time` | The median time spent at home for all devices at this location for this time period, in minutes |
+| `completely_home_prop_7dav` | Offers a 7-day trailing window average of the `completely_home_prop`. |
+| `full_time_work_prop_7dav` | Offers a 7-day trailing window average of the`full_time_work_prop`. |
+| `part_time_work_prop_7dav` | Offers a 7-day trailing window average of the`part_time_work_prop`.|
+| `median_home_dwell_time_7dav` | Offers a 7-day trailing window average of the `median_home_dwell_time`.|
 
 After computing each metric on the census block group (CBG) level, we aggregate
 to the county-level by taking the mean over CBGs in a county to obtain the value
@@ -42,6 +46,11 @@ doing so, we make the simplifying assumption that each CBG contributes an iid
 observation to the county-level distribution. `n` also serves as the sample
 size. The same method is used for aggregation to states.
 
+SafeGraph's signals measure mobility each day, which causes strong day-of-week effects:
+weekends have substantially different values than weekdays. Users interested in long-term
+trends, rather than mobility on one specific day, may prefer the `7dav` signals since
+averaging over the preceding 7 days removes these day-of-week effects.
+
 ## Lag
 
 SafeGraph provides this data with a three-day lag, meaning estimates for a