Add documentation for CookieManager defualts

Documents the defualt settings of acceptCookies and acceptFileSchemeCookies.

The current defaults:
                            <L       >=L
acceptCookies           |    y   |    y   |
acceptThirdPartyCookies |    y   |    n   |
acceptFileCookies       |    n   |    n   |

Bug: 11575158
Change-Id: Ia6cb71bf80608cb2ea8049192f33616e57c15801