fix: update error message about scalar slide_computations

R/slide.R

@@ -357,8 +357,8 @@ epi_slide_one_group <- function(
   # Returned values must always be a scalar vector or a data frame with one row.
   if (any(vctrs::list_sizes(slide_values_list) != 1L)) {
     cli_abort(
-      "The slide computations must either (a) output a single element/row each or
-        (b) one element/row per appearance of the reference time value in the local window.",
+      "epi_slide: slide computations must return a single element (e.g. a scalar value, a single data.frame row,
+      or a list).",
       class = "epiprocess__invalid_slide_comp_value"
     )
   }