Docs: Add workaround notice when biometry is canceled in non-standard way

[hvge]

We should add a notice, or warning to the iOS documentation, that application may add a custom workaround to handle situations where Touch/Face ID is displayed, and the user cancels the biometric authentication in the non-standard way. That may happen when user hits the power button, home button (Touch ID), or exits the application in other way (like swipe on Face ID devices). The problem is that at the SDK level, we cannot distinguish between a regular biometric authentication failure and this kind of cancelation. For both cases, iOS returns the same error errSecAuthFailed.

Related to #271 #9

[hvge]

Version 1.7.5 has an improved authenticateUsingBiometry() function that can be used as a workaround for this issue. See our documentation for more info.

The problem will be properly fixed in 1.8.x release.