Clarify NSSP documentation, especially 'HSA' semantics

docs/api/covidcast-signals/nssp.md

@@ -43,6 +43,7 @@ As of May 2024, NSSP received data from 78% of US EDs.
 
 The percent visits signals are calculated as a fraction of visits at facilities reporting to NSSP, rather than all facilities in the area.
 `county`, `state` and `nation` level data is reported as-is from NSSP, without modification, while `hhs`, `hrr` and `msa` are estimated by Delphi.
+State and HSA-level values are calculated and published by NSSP; County level values are not published individually, but are approximations copied from the HSA the county is in (every county in an HSA will have identical values).
 
 ### Geographic weighting
 As the original data is a percentage and raw case counts are not available, `hrr`,`msa`, and `hhs` values are computed from county-level data using a weighted mean. Each county is assigned a weight equal to its population in the last census (2020). Unreported counties are implicitly treated as having a weight of 0 or a value equal to the group mean.
@@ -69,6 +70,8 @@ The following states report no data through NSSP at the county level: CA, WA, AK
 
 South Dakota, Missouri, and territories report no data through NSSP at the state level.
 
+The only completely non-reporting state is Missouri.
+
 
 ## Lag and Backfill
 
@@ -84,6 +87,9 @@ Counties with `NA` values are as originally reported in the dataset from which t
 
 ## Limitations
 
+As noted above, only state and HSA-level values are calculated and published by NSSP; County level values are not published individually, but are approximations copied from the HSA the county is in (every county in an HSA will have identical values).
+The HSA (Health Service Area) definitions used are known as ["NCI Modified"](https://seer.cancer.gov/seerstat/variables/countyattribs/hsa.html).
+
 There is substantial missingness at the county level. This tends to impact more rural and lower population locations. See the [missingness section](#missingness) for more information.
 
 Not all counties contain reporting EDs, including in states where NSSP reports state-level data.
@@ -104,4 +110,4 @@ Some low population counties occasionally report outliers, e.g. 33.33%, 50%, 100
 This source is derived from the CDC's [Respiratory Virus Response NSSP Emergency Department Visit Trajectories dataset](https://data.cdc.gov/Public-Health-Surveillance/2023-Respiratory-Virus-Response-NSSP-Emergency-Dep/rdmq-nq56/about_data).
 There is another version of the dataset that includes [state data only](https://data.cdc.gov/Public-Health-Surveillance/2023-Respiratory-Virus-Response-NSSP-Emergency-Dep/7mra-9cq9/about_data).
 
-This data was originally published by the CDC, and is made available here as a convenience to the forecasting community under the terms of the original license, which is [U.S. Government Public Domain](https://www.usa.gov/government-copyright).
+This data was originally published by the CDC, and is made available here as a convenience to the forecasting community under the terms of the original license, which is [U.S. Government Public Domain](https://www.usa.gov/government-copyright).