Autocomplete: Clarify 'isDebounced' setting limitation

[Mamaduka]

What?

This is a follow-up to #76995 (comment).

PR adds clarification for isDebounced settings for autocompleters.

Testing Instructions

None. It's just a documentation update. We can also ignore changelog entry for this change.

Use of AI Tools

None.

[github-actions]

Flaky tests detected in 1ae3494.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/24034167226
📝 Reported issues:

• [Flaky Test] post title and content are editable and blocks can be inserted #76524 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js
• [Flaky Test] should show correct homepage actions based on current homepage or posts page #68892 in /test/e2e/specs/site-editor/homepage-settings.spec.js

[ntsekouras]

Thank you!

[github-actions]

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Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: ntsekouras <ntsekouras@git.wordpress.org>

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