docs: n_units_reporting is category-conditional, not a response rate

Closes uscogdata#36. It counts governments with rows for the requested
category, so a surveyed government that genuinely spends nothing there
is indistinguishable from one never surveyed. In FY2022, a complete
census year, Georgia reports 393 of 567 cities for Police -- the gap is
cities that contract to the sheriff.

Documents the comparison that IS valid: same category, census year vs
sample year.

man/cog_peer_compare.Rd

@@ -107,3 +107,23 @@ call. Those summary rows are quantiles **within each category**, not
 quantiles of each peer's total — see the `@return` section before summing
 them.
 }
+\section{Reading `coverage`}{
+
+`provenance$coverage` reports `n_units_reporting` against
+`n_units_expected` per year. **`n_units_reporting` is category-conditional:
+it counts cohort members with rows for the category you asked for, not
+cohort members collected that year.** A government that was surveyed and
+genuinely spends nothing in that category is indistinguishable here from one
+that was never surveyed.
+
+The ratio is therefore **not a response rate** and must not be used as one.
+In FY2022 — a complete census year — Georgia reports 393 of 567 cities for
+`category = "Police"`; the 174-city gap is overwhelmingly cities that
+contract policing to the county sheriff, not non-response.
+
+The comparison that *is* valid is the same category across a census year
+(ending in 2 or 7) and a sample year, where the real-zero component is
+roughly constant and the difference reflects the survey cycle. `is_census_year`
+marks which is which.
+}
+