Docs: Added missing parameter (#1774)

This adds a missing parameter to the `Prism.highlight` documentation.

extending.html

@@ -202,16 +202,18 @@ <h2>Parameters</h2>
 	</section>
 
 	<section id="highlight">
-		<h1><code>Prism.highlight(text, grammar)</code></h1>
+		<h1><code>Prism.highlight(text, grammar, language)</code></h1>
 		<p>Low-level function, only use if you know what you’re doing.
-		It accepts a string of text as input and the language definitions to use, and returns a string with the HTML produced.</p>
+		It accepts a string of text as input, the language definitions to use, and the name of the language, and returns a string with the HTML produced.</p>
 
 		<h2>Parameters</h2>
 		<dl>
 			<dt>text</dt>
 			<dd>A string with the code to be highlighted.</dd>
 			<dt>grammar</dt>
 			<dd>An object containing the tokens to use. Usually a language definition like <code>Prism.languages.markup</code></dd>
+			<dt>language</dt>
+			<dd>The name of the language definition passed to <code>grammar</code>. E.g. <code>markup</code> or <code>javascript</code></dd>
 		</dl>
 
 		<h2>Returns</h2>