add a note about notation back

Author: michaelficarra

spec.html

@@ -588,6 +588,7 @@ <h1>Terminal Symbols</h1>
         <p>In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal symbol.</p>
         <p>Terminal symbols come in two other forms:</p>
         <ul>
+          <li>In the lexical and RegExp grammars, Unicode code points without a conventional printed representation are instead shown in the form "&lt;U+0000 (NULL)>" where `0000` is 4 to 6 hexits representing the code point in hexadecimal notation and `NULL` is the code point name.</li>
           <li>In the syntactic grammar, certain terminal symbols (e.g. |IdentifierName| and |RegularExpressionLiteral|) are shown in italics, as they refer to the nonterminals of the same name in the lexical grammar.</li>
         </ul>
       </emu-clause>