docs: clarify when index property in ViewToken can be null

[supunyasanthaofficial]

Fixes #3637

Summary

Updates the ViewToken documentation to explain why the index property is optional.

The index can be null when the viewable element is not a regular item from the data array. For example, this can happen when a section header or footer becomes viewable in a SectionList.

The same update has also been backported to the versioned docs for 0.77 through 0.87.

Test Plan

• Ran Prettier on the modified files.
• Verified ESLint passes:
  npx eslint docs/viewtoken.md website/versioned_docs/*/viewtoken.md
• Verified the Docusaurus build succeeds:
  yarn --cwd website docusaurus build

[Simek]

Thanks for providing a wider context on this change, and adding an explanation to the docs! 👍

[Simek]

Wording tweaks.