mention "modern" fake timers implementation in docs

[PavelPolyakov]

Summary

Docs update. It's currently difficult to find out, that it's even possible to try this new implementation. I've accidentally met it in the release notes.

Test plan

none

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[jeysal]

Thanks! @SimenB does this go far enough or would you want this guide to explicitly recommend using 'modern'?

[SimenB]

I think we should recommend it. It'll be the default in the next major, and the sooner people migrate the better 😀

[jeysal]

I wonder though: What will happen after the default changes? Lots of people with unnecessary 'modern' in their codebase :|
If the change is non-breaking for the majority of people (no idea if it is) maybe it's fine to not give the recommendation to everyone now?

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[SimenB]

Lots of people with unnecessary 'modern' in their codebase :|

Not sure I think this is an issue. It's a search&replace away. Can probably add a lint rule or codemod that can fix it (we probably want one so FB can go the other way - add 'legacy' to all without arguments)

If the change is non-breaking for the majority of people (no idea if it is)

It should be

[codecov-io]

Codecov Report

Merging #10022 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10022   +/-   ##
=======================================
  Coverage   64.00%   64.00%           
=======================================
  Files         297      297           
  Lines       12528    12528           
  Branches     3093     3093           
=======================================
  Hits         8018     8018           
  Misses       3860     3860           
  Partials      650      650           

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[facebook-github-bot]

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[SimenB]

Landed in #12317

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