Block Editor: Clarify logic for 'directInsert' inner blocks setting

[Mamaduka]

What?

Related #77858.

PR clarifies docs for directInsert inner blocks setting and how it works based on other block configs.

Testing Instructions

Confirm updated docs make sense.

Testing Instructions for Keyboard

Same.

Use of AI Tools

Grammarly, I'm bad at grammar.

[github-actions]

Flaky tests detected in 34e550d.
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/25204088951
📝 Reported issues:

• [Flaky Test] Text blocks sync modifications between users #77329 in /test/e2e/specs/editor/collaboration/collaboration-block-gauntlet.spec.ts
• [Flaky Test] post title and content are editable and blocks can be inserted #77720 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 #77385 in /test/e2e/specs/site-editor/homepage-settings.spec.js