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## DESCRIBE YOUR PR
Related to getsentry/sentry#90993 , where we
added the prefix `data.` to all unknown User Context keys, making them
stand out in the UI.

It was confusing to some users that the UI showed the User data in a
"prettified" manner, not matching the expected keys sent from the SDKs,
so I'm adding some warnings/notes to the develop docs to clear this up.

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## PRE-MERGE CHECKLIST

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- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
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---------

Co-authored-by: Karl Heinz Struggl <kahest@users.noreply.github.com>
Co-authored-by: Alex Krawiec <alex.krawiec@sentry.io>

Fixes getsentry/sentry-native#1225

For User Context, SDKs are
[expected](https://develop.sentry.dev/sdk/data-model/event-payloads/user/)
to send the default fields in lowercase (e.g. `username`, `email`,
`ip_address`, ...) which get rendered nicely (`Username`, `Email`, `IP
Address`, ...) in the Sentry UI. But, when they send us a key
like`Email` this will look the same in the UI (but will not be treated
the same). Hence I propose this fix which marks keys from the user
context's `data` section as `data.X`.

(this is just a suggestion; maybe there is a cleaner way to mark these
items as "data" that has less visual clutter, so I'm open to feedback 😄
)

```json
"user": {
    "id": "42",
    "email": "john.doe@email.com",
    "ip_address": "127.0.0.1",
    "username": "John Doe",
    "sentry_user": "id:42",
    "data": {
        "Email": "jane.doe@email.com",
        "Username": "Jane Doe"
    }
}
```
currently rendered as 


![image](https://github.com/user-attachments/assets/caef6671-e604-44fd-ad53-a1796f0f3a4b)

newly rendered as 

![image](https://github.com/user-attachments/assets/48226087-b77f-4bcb-969c-331942198759)