Document LAContext reuse behavior

[hvge]

Once we allowed use LAContext object instead of prompt string for the biometric authentications, we opened a gate that allows application to use such context for multiple times. Basically, application can pre-authorize its own context and re-use it for multiple times for the biometric signature.

We should mention in documentation that this usage is strongly not recommended, due to the following reasons:

• LAContext can be reused for a very long time and the biometric dialog is not displayed. The exact time depends on iOS implementation, but can be longer than 5 minutes and shorted than 10 minutes (validated by our experiments)
• It would cause a legislative problems in case that application signs a critical data (like payment). It would be problematic to prove that user authenticated such request, or it's just a bug in the application.