CSP 1.1: Add non-normative language for extensions.

After discussion outside the WG, on the list[1] and on the
2014-02-16 call[2], the WG decided to revisit the change in
[3]. The language in this patch seems like a compromise that
everyone can accept.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Feb/0098.html
[2]: http://www.w3.org/2014/02/26-webappsec-minutes.html
[3]: cbfaa8e

specs/content-security-policy/csp-specification.dev.html

@@ -913,6 +913,10 @@ <h3>Processing Model</h3>
        usurp the resource's privileges that have been restricted in this
        way.</p>

        <p>Note that user agents may allow users to modify or bypass policy
        enforcement through user preferences, bookmarklets, third-party
        additions to the user agent, and other such mechanisms.</p>

        <p>To <dfn id="monitor">monitor</dfn> a policy, the user agent MUST
        <a href="#parse-a-policy">parse the policy</a> and monitor each of
        the directives contained in the policy.</p>