Document the nickname property when registering a credential. (#97)

Author: Jonas Hendrickx

passwordless/src/main/java/dev/passwordless/android/rest/contracts/register/RegisterCompleteRequest.kt

@@ -8,7 +8,8 @@ import com.google.gson.annotations.SerializedName
  *
  * @property session The session ID for the registration.
  * @property response The response from the authenticator.
- * @property nickname The nickname of the user.
+ * @property nickname The nickname of a credential could represent the location where the credential
+ * is stored, i.e. "YubiKey Work" or "Marc's Laptop".
  * @property origin The origin of the request.
  * @property rpId The identifier of the relying party.
  */