Clarify getAvailableBiometrics documentation regarding iOS permissions

Author: anirudhsharma

packages/local_auth/local_auth/lib/src/local_auth.dart

@@ -88,7 +88,13 @@ class LocalAuthentication {
   /// fail over to device credentials.
   Future<bool> isDeviceSupported() async => LocalAuthPlatform.instance.isDeviceSupported();
 
-  /// Returns a list of enrolled biometrics.
+  /// Returns a list of the biometrics that the app can currently use for
+  /// authentication.
+  ///
+  /// Note that this may not include all biometrics enrolled on the device.
+  /// For example, on iOS, biometrics may be enrolled on the device but
+  /// remain unavailable to the app if the user has not granted the required
+  /// permissions to use them.
   Future<List<BiometricType>> getAvailableBiometrics() =>
       LocalAuthPlatform.instance.getEnrolledBiometrics();
 }