Describe what happens to invalid email addresses in feedback (#14103)

Resolves getsentry/sentry-dotnet#4284

See:
- getsentry/sentry-dotnet#4284 (comment)

Author: jamescrosswell

docs/platforms/dotnet/common/user-feedback/index.mdx

@@ -24,6 +24,12 @@ var eventId = SentrySdk.CaptureMessage("An event that will receive user feedback
 SentrySdk.CaptureFeedback("It broke.", "[email protected]", "The User", associatedEventId: eventId);
 ```
 
+<Alert type="info">
+
+Note that Sentry will reject and drop any feedback events where the email address is invalid. To avoid this, the Sentry SDK for .NET removes invalid email addresses before sending feedback. Ideally you should validate the email address before calling this API.
+
+</Alert>
+
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