Add more documentations for GIDUserAuth.

GoogleSignIn/Sources/GIDUserAuth_Private.h

@@ -22,6 +22,9 @@ NS_ASSUME_NONNULL_BEGIN
 @interface GIDUserAuth ()
 
 // Private initializer for |GIDUserAuth|.
+// @param user The current GIDGoogleUser.
+// @param severAuthCode The one-time authorization code for backend to exchange
+//     access and refresh tokens.
 - (instancetype)initWithGoogleUser:(GIDGoogleUser *)user
                     serverAuthCode:(nullable NSString *)serverAuthCode;
 

GoogleSignIn/Sources/Public/GoogleSignIn/GIDUserAuth.h

@@ -29,8 +29,10 @@ NS_ASSUME_NONNULL_BEGIN
 /// An OAuth2 authorization code for the home server.
 @property(nonatomic, readonly, nullable) NSString *serverAuthCode;
 
+/// Unsupported.
 + (instancetype)new NS_UNAVAILABLE;
 
+/// Unsupported.
 - (instancetype)init NS_UNAVAILABLE;
 
 @end