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.

man/cog_spending.Rd

@@ -22,7 +22,15 @@ cog_spending(
 \item{years}{Integer vector of years.}
 
 \item{category}{Character vector of category names (from
-`summary_categories.category`), or `NULL` for all categories.}
+`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
+`expenditure_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
+operating-expenditure total.}
 
 \item{per_capita}{If `TRUE`, adds `amt_per_capita_nominal` (and
 `amt_per_capita_real` when `adjust_to_year` is set) using the per-year