Improve Enum.with_index/2 docs (#13671)

Author: vanderhoop

lib/elixir/lib/enum.ex

@@ -3921,12 +3921,10 @@ defmodule Enum do
   Returns the `enumerable` with each element wrapped in a tuple
   alongside its index or according to a given function.
 
-  May receive a function or an integer offset.
-
-  If an `offset` is given, it will index from the given offset instead of from
+  If an `offset` integer is given, it will index from the given offset instead of from
   zero.
 
-  If a `function` is given, it will index by invoking the function for each
+  If an `offset` function is given, it will index by invoking the function for each
   element and index (zero-based) of the enumerable.
 
   ## Examples