Mention the OpenID provider restriction feature in our design document
Mention the OpenID provider restriction feature in our design document

This is an important feature in our security design, as it helps to
reduce the attack surface available due to the use of OpenID.

Signed-off-by: Shawn O. Pearce <f3ea74d906fa9fe97c1fef6bad9cb871485c7045@google.com>

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