feat(apple): Start the SDK before calling setUser (#12798)

Add a comment to the setUser code snipped to point out users have to
start the SDK before.

Author: philipphofmann

platform-includes/enriching-events/set-user/apple.mdx

@@ -3,6 +3,7 @@ import Sentry
 
 let user = User()
 user.email = "[email protected]"
+// Start the SDK before setting the user, otherwise it will be ignored.
 SentrySDK.setUser(user)
 ```
 
@@ -11,5 +12,6 @@ SentrySDK.setUser(user)
 
 SentryUser *user = [[SentryUser alloc] init];
 user.email = @"[email protected]";
+// Start the SDK before setting the user, otherwise it will be ignored.
 [SentrySDK setUser:user];
 ```