docs: Update use-focus-effect.md to explain cleanup function

[fitterman]

Added explanation of what the cleanup function is and how the user creates it.

READ ME PLEASE!

TL;DR: Make sure to add your changes to versioned docs

Thanks for opening a PR!

The docs cover several versions of react-navigation, and in some cases there are several files (for version 1, version 2 and etc.) that all describe a single page of the docs (eg. "Getting Started").

Please make sure that the edit you're making in docs/file-you-edited.md is also included in the file for the correct version, eg. /versioned_docs/version-3.x/file-you-edited.md for version 3. If such file doesn't exist, please create it. :+1:

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