Add note about time freezing in specs

What does this MR do and why?

With the introduction of let_it_be some specs can be flaky if they are freezing time since around each only freezes the spec activity itself, not spec setup. Using fuzzy matchers such as be_like_time can mitigate it, but external factors (speed of specs, etc) can cause the specs to be flaky.

This MR adds a small note about this to our docs.

See #413223 (closed) for an example.

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.