Update documentation regarding auth-bound keys.

When a key requires user authentication and one of the authentication
methods permitted is the device's screen lock credentials, the
root SID is added as an authenticator, and change of biometrics
enrollment will not invalidate the key.

Bug: 275900161
Test: m docs
Change-Id: I180f28883a5ac62e8bfa0b0596396085ff676637
1 file changed