WebView: deprecate 'Secure' cookies for insecure schemes

No change to logic, only docs. This announces 'Secure' cookies are
deprecated for insecure URL schemes (only "https://" is considered
secure). This doesn't mention target SDK, because apps should follow
this guidance for all WebView versions, target SDKs, and OS levels.

Bug: 149589092
Test: m offline-sdk-docs -j4
Change-Id: I07c2b5341588d354f7f8219ce71a3d2ca04bc982