Mark input purposes list as non-normative

Closes #3777

guidelines/input-purposes.html

@@ -1,4 +1,4 @@
-<section id="input-purposes">
+<section class="informative" id="input-purposes">
 	<h2>Input Purposes for User Interface Components</h2>
 	<p>This section contains a listing of common <a>user interface component</a> input purposes. The terms below are not keywords that must be used, but instead represent purposes that must be captured in the taxonomy adopted by a webpage. Where applicable, authors mark up controls with the chosen taxonomy to indicate the semantic purpose. This provides the potential for user agents and assistive technologies to apply personalized presentations that can enable more people to understand and use the content.</p>