Clarify JSX usage in notifications expanded viewdocs: add note explai…

[knisaci]

…ning JSX requirement for expanded notification view

The notifications page switches from plain JS to JSX in the expanded view example without explanation, which can confuse new developers.

Added a sentence clarifying that the content field requires JSX components such as Box and Text, with a link to the Custom UI docs for context.

Fixes #2895

Description

Issue(s) fixed

Fixes #

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Checklist

• If this PR updates or adds documentation content that changes or adds technical meaning, it has received an approval from an engineer or DevRel from the relevant team.
• If this PR updates or adds documentation content, it has received an approval from a technical writer.

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• I've read the contribution guidelines.
• I've created a new issue (or assigned myself to an existing issue) describing what this PR addresses.

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Note

Low Risk
Low risk documentation-only change that clarifies usage; no runtime behavior or APIs are modified.

Overview
Clarifies the notifications expanded-view docs by explicitly stating that the content field must be authored using JSX components (e.g., Box, Text) and links readers to the Custom UI documentation for background before following the example.

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