[ESQL] Docs clarify counts are approximate in count_distinct docs (#135600)

need to make this visible in Kibana inline docs

Author: leemthompo

docs/reference/query-languages/esql/_snippets/functions/description/count_distinct.md

@@ -74,6 +74,8 @@ public class CountDistinct extends AggregateFunction implements OptionalArgument
     @FunctionInfo(
         returnType = "long",
         description = "Returns the approximate number of distinct values.",
+        note = "[Counts are approximate](/reference/query-languages/esql/functions-operators/"
+            + "aggregation-functions.md#esql-agg-count-distinct-approximate).",
         appendix = """
             ### Counts are approximate [esql-agg-count-distinct-approximate]