Editorial: Update abstract line of JSON.stringify section (#1345)

- Previously did not account for any obvious mention of non-string
  return value.

spec.html

@@ -35867,7 +35867,7 @@ <h1>Runtime Semantics: InternalizeJSONProperty ( _holder_, _name_ )</h1>
 
     <emu-clause id="sec-json.stringify">
       <h1>JSON.stringify ( _value_ [ , _replacer_ [ , _space_ ] ] )</h1>
-      <p>The `stringify` function returns a String in UTF-16 encoded JSON format representing an ECMAScript value. It can take three parameters. The _value_ parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or *null*. The optional _replacer_ parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional _space_ parameter is a String or Number that allows the result to have white space injected into it to improve human readability.</p>
+      <p>The `stringify` function returns a String in UTF-16 encoded JSON format representing an ECMAScript value, or *undefined*. It can take three parameters. The _value_ parameter is an ECMAScript value, which is usually an object or array, although it can also be a String, Boolean, Number or *null*. The optional _replacer_ parameter is either a function that alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an inclusion list for selecting the object properties that will be stringified. The optional _space_ parameter is a String or Number that allows the result to have white space injected into it to improve human readability.</p>
       <p>These are the steps in stringifying an object:</p>
       <emu-alg>
         1. Let _stack_ be a new empty List.