Add documentation for credential type.

Author: kinyoklion

packages/common_client/lib/src/config/defaults/credential_type.dart

@@ -1,4 +1,9 @@
+/// Represents the credential type used by the SDK. The credential type is
+/// determined by the platform the SDK is running on.
 enum CredentialType {
+  /// The SDK is using a mobile key credential.
   mobileKey,
+
+  /// The SDK is using a client-side ID.
   clientSideId,
 }