Change text for Security->Site Policy URL to include warning about Internet Explorer not displaying site policy file located on different domain.

Background:
By default, Internet Explorer 6 & 7 (not sure about other versions) will display a blank screen if the Site Policy URL points to a file located on a different domain. Text should warn admins of this fact, so they don't do it!!!

Suggested change:

Security->Site Policy URL text should read something like this:

If you have a site policy that all users must see and agree to before using this site, then specify the URL to it here, otherwise leave this field blank. The URL can point to anywhere - one convenient place would be a file in the site files. eg http://yoursite/file.php/1/policy.html. (NOTE: By default, Internet Explorer may display a blank screen if the Site Policy URL is located on a different domain than the domain that Moodle is installed on. To prevent problems for users, it is suggested that the Site Policy URL by on the same domain.)

See this issue for more details on the problem:
http://tracker.moodle.org/browse/MDL-14609