Add a [Deprecated] mark to the API reference (#12287)

Author: renkelvin

FirebaseAuth/Sources/Public/FirebaseAuth/FIRAuth.h

@@ -377,8 +377,8 @@ NS_SWIFT_NAME(Auth)
                completion:(nullable void (^)(NSError *_Nullable error))completion;
 
 /** @fn fetchSignInMethodsForEmail:completion:
-    @brief Fetches the list of all sign-in methods previously used for the provided email address.
-   This method returns an empty list when [Email Enumeration
+    @brief [Deprecated] Fetches the list of all sign-in methods previously used for the provided
+   email address. This method returns an empty list when [Email Enumeration
    Protection](https://cloud.google.com/identity-platform/docs/admin/email-enumeration-protection)
    is enabled, irrespective of the number of authentication methods available for the given email.
     @param email The email address for which to obtain a list of sign-in methods.

FirebaseAuth/Sources/Public/FirebaseAuth/FIRUser.h

@@ -131,8 +131,8 @@ NS_SWIFT_NAME(User)
 - (instancetype)init NS_UNAVAILABLE;
 
 /** @fn updateEmail:completion:
-    @brief Updates the email address for the user. On success, the cached user profile data is
-   updated. Throws FIRAuthErrorCodeInvalidCredentials error when [Email Enumeration
+    @brief [Deprecated] Updates the email address for the user. On success, the cached user profile
+   data is updated. Throws FIRAuthErrorCodeInvalidCredentials error when [Email Enumeration
    Protection](https://cloud.google.com/identity-platform/docs/admin/email-enumeration-protection)
    is enabled.
     @remarks May fail if there is already an account with this email address that was created using