[DOCS] Mention option to return String in sort context (#76105)

* Painless: Mention option to return String in sort context

* Adjust wording

docs/painless/painless-contexts/painless-sort-context.asciidoc

@@ -20,8 +20,9 @@ Use a Painless script to
 
 *Return*
 
-`double`::
-        The score for the specified document.
+`double` or `String`::
+        The sort key. The return type depends on the value of the `type` parameter
+        in the script sort config (`"number"` or `"string"`).
 
 *API*
 
@@ -58,4 +59,4 @@ GET /_search
   }
 }
 ----
-// TEST[setup:seats]
+// TEST[setup:seats]