Add close( ) method to Apple drain doc (#13492)

## DESCRIBE YOUR PR
Updating the Apple drain doc to include the `close ( )` method. 

Liked to #13468

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## PRE-MERGE CHECKLIST

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- [ ] Checked Vercel preview for correctness, including links
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---------

Co-authored-by: Shannon Anahata <[email protected]>

Author: sfanahata

platform-includes/configuration/drain-example/apple.mdx

@@ -1 +1,7 @@
 The Apple SDK automatically stores the Sentry events on the device's disk before shutdown.
+
+There are times where you may need to manually call `close()` to ensure that events are sent before the app terminates. For example, if an app user revokes some privacy consent and the app can't use Sentry anymore, calling `close()` will still send the events to Sentry.
+
+```swift
+SentrySDK.close()
+```