Add more descriptive comments for the Validators on TokenValidationParameters

[mafurman]

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Our current comments are not very helpful, and they should be modified to be more descriptive of the purpose of each delegate.

"The AudienceValidator delegate, if set, will be called by the runtime instead of default audience validation. This extensibility point provides the user with an opportunity to validate the audiences found in the security token in some custom way. If this extensibility is not provided, the audiences found in the token will be checked against TokenValidationParameters.Valid Audience and TokenValidationParameters.ValidAudiences."

[TimHannMSFT]

I'll opportunistically take this on next time I'm in Wilson.