docs(completeHTMLDocument): Change completeHTMLDocument comment (#610)

Fix completeHTMLDocument comment to reflect its actual behavior

src/subParsers/completeHTMLDocument.js

@@ -1,5 +1,5 @@
 /**
- * Turn Markdown link shortcuts into XHTML <a> tags.
+ * Create a full HTML document from the processed markdown
  */
 showdown.subParser('completeHTMLDocument', function (text, options, globals) {
   'use strict';