Update development docs related to let_it_be freeze

What does this MR do and why?

This MR updates the documentation around using let_it_be with freeze, to help mitigate state leaks.

This update was prompted based on a comment on a recent MR !132138 (comment 1565546846) which points out the correct way of freezing an object while using let_it_be from the test-prof docs.

Screenshots or screen recordings

No visible change

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.