docs: add note on installing `server-only` in data security guide (#92445)
## What?
Improve alignment between the following documentation sections to ensure
clearer and more consistent guidance:
- Data Security:
https://nextjs.org/docs/app/guides/data-security#preventing-client-side-execution-of-server-only-code
- Preventing Environment Poisoning:
https://nextjs.org/docs/app/getting-started/server-and-client-components#preventing-environment-poisoning
**Related**: https://github.com/vercel/next.js/discussions/91180
## Why?
The current documentation presents overlapping concepts with slight
inconsistencies, which can lead to confusion for readers.
## Changes
- Align explanations across both sections
- Improve overall consistency in guidance
Docs only change.
@icyJoseph
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Co-authored-by: Joseph <sephxd1234@gmail.com>
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>