chore: clarify user context UI and interface (#13620)

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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

*Make sure you've checked the following before merging your changes:*

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

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

Author: 3 people

develop-docs/sdk/data-model/event-payloads/user.mdx

@@ -10,6 +10,10 @@ You should provide at least one of `id`, `email`, `ip_address`,
 affected by one issue, for example. Sending a user that has none
 of these attributes and only custom attributes is valid, but not as useful.
 
+<Alert title="Note" level="warning">
+These keys are case-sensitive, any mistyped keys will be processed as custom attributes.
+</Alert>
+
 ## Attributes
 
 `id`
@@ -31,6 +35,10 @@ of these attributes and only custom attributes is valid, but not as useful.
 All other keys are stored as extra information but not specifically processed by
 Sentry.
 
+<Alert title="Note">
+The titles of these well-known attributes are prettified in the UI. For example, `ip_address` is shown as `IP Address`. Other keys are shown as they were sent, with the prefix `data.`.
+</Alert>
+
 `geo`
 
 : An optional object describing the [geographical location](#geographical-location) of the end user or device, this object is automatically inferred by Relay if `ip_address` is provided.