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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

❗ 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

@@           Coverage Diff           @@
##             main    #2570   +/-   ##
=======================================
  Coverage   96.02%   96.02%           
=======================================
  Files          40       40           
  Lines        1912     1912           
=======================================
  Hits         1836     1836           
  Misses         76       76           

Flag	Coverage Δ
unit	96.02% <ø> (ø)

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