Add a notice that the Fake Timers API doc is incomplete

[fatso83]

Purpose (TL;DR) - mandatory

This is basically linking to the original Fake Timers project as an excuse for not keeping the documented API in sync.

This is a case where the community TypeScript effort at DefinitelyTyped is doing a much better job than us, as they simply import the fake-timers types in the Sinon definitions.

How to verify - mandatory

1. Check out this branch
2. npm install

Checklist for author

• [ ] npm run lint passes
• [ ] References to standard library functions are cached.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (93db3ef) 96.02% compared to head (bff296c) 96.02%.

:exclamation: Current head bff296c differs from pull request most recent head 575d90d. Consider uploading reports for the commit 575d90d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2570 +/- ## ======================================= Coverage 96.02% 96.02% ======================================= Files 40 40 Lines 1912 1912 ======================================= Hits 1836 1836 Misses 76 76 ``` | [Flag](https://app.codecov.io/gh/sinonjs/sinon/pull/2570/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sinonjs) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/sinonjs/sinon/pull/2570/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sinonjs) | `96.02% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=sinonjs#carryforward-flags-in-the-pull-request-comment) to find out more.

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