Clarify ref callback cleanup behavior in common ref docs

[smg99]

Fixes #7811

Summary

• Clarify the ref callback behavior when callbacks change between renders
• Explicitly describe the cleanup-function path as the primary behavior
• Clarify that passing null to the old callback is legacy fallback behavior when no cleanup is returned

Why

Issue #7811 points out ambiguity in the current wording, especially around when React calls a previous callback with null. This change aligns wording with current React 19 cleanup semantics while preserving backwards-compatibility notes.

Validation

• Ran yarn lint
• Ran yarn lint-heading-ids

[smg99]

Requesting review for this wording clarification.

This updates the ref callback section to emphasize the cleanup-function path first, and clarifies that null is the legacy fallback when no cleanup function is returned.

Happy to refine wording further if maintainers prefer a different phrasing.

[github-actions]

Size changes

Details

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