feat: accept category = 'All Categories' on cog_spending/cog_revenue

Returns one summed row per (year, govid, subtype) across every
category in the requested concept's subtype scope, so a caller never
sums categories client-side and cannot sum the wrong scope.

Combining it with other category names is an error rather than a
silent partial sum.

R/revenue.R

@@ -8,6 +8,16 @@
 #' multiplies by 1000 and records the conversion in `provenance`).
 #'
 #' @inheritParams cog_spending
+#' @param category Character vector of category names (from
+#'   `summary_categories.category`), or `NULL` for all categories broken out
+#'   one row each. The reserved value `"All Categories"` instead returns a
+#'   single summed row per `(year, canonical_govid, subtype)`, covering every
+#'   category inside the requested concept's subtype scope. It cannot be
+#'   combined with other category names, and it is not the same thing as
+#'   `revenue_concept = "total"`: the concept chooses which subtypes are in
+#'   scope, `"All Categories"` chooses whether rows inside that scope are
+#'   broken out or summed. Combine with `subtype = "operations"` for an
+#'   own-source revenue total.
 #' @param revenue_concept Which of Census's two published revenue concepts to
 #'   return. Concepts are defined as sets of the crosswalk's `revenue_subtype`
 #'   values -- never as item-code first letters, which cannot classify