Add documentation for `SkipTermsOfUse` policy.

The `SkipTermsOfUse` policy is available in Firefox 138+. If true, don't display the Firefox Terms of Use and Privacy Notice upon startup. You represent that you accept and have the authority to accept the Terms of Use on behalf of all individuals to whom you provide access to this browser.

https://www.mozilla.org/about/legal/terms/firefox/
https://www.mozilla.org/privacy/firefox/