Document PowerAuthKeychainConfiguration

[kober32]

We encountered a client, who configured PowerAuth configuration with everything, even PowerAuthKeychainConfiguration. And they configured it wrong in a way that the data was not persisted between runs.

Our docs are missing what each configuration items are meant for (like keychainInstanceName_Biometry).

Plus we should also advise with a warning not to configure it if not necessary and that the default values should be enough unless you have some specific task you're solving.