Add information about after clause timeout limit (#5942)

Source: http://erlang.org/doc/reference_manual/expressions.html#id80928

lib/elixir/lib/kernel/special_forms.ex

@@ -1900,8 +1900,8 @@ defmodule Kernel.SpecialForms do
       end
 
   The `after` clause can be specified even if there are no match clauses.
-  The timeout value given to `after` can be a variable; two special
-  values are allowed:
+  The timeout value given to `after` can be any expression evaluating to
+  one of the allowed values:
 
     * `:infinity` - the process should wait indefinitely for a matching
       message, this is the same as not using a timeout
@@ -1909,6 +1909,10 @@ defmodule Kernel.SpecialForms do
     * `0` - if there is no matching message in the mailbox, the timeout
       will occur immediately
 
+    * positive integer smaller than `4_294_967_295` (`0xFFFFFFFF`
+      in hex notation) - it should be possible to represent the timeout
+      value as an unsigned 32-bit integer.
+
   ## Variables handling
 
   The `receive/1` special form handles variables exactly as the `case/2`