gh-118392: Add note about random.random for multi thread app

[corona10]

• Issue: Dramatic slowdown with random.random on free-threading build. #118392

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📚 Documentation preview 📚: https://cpython-previews--118396.org.readthedocs.build/

[colesbury]

Let's move this up to the module level, perhaps between the security warning and the "see also". The advice is not limited this function.

I think maybe a .. note:: is more appropriate. A red "warning" seems a bit severe to me.

For the text, something like:

The global random number generator and instances of :class:`Random` are thread-safe. However, in the free-threaded build, concurrent calls to the global generator or to the same instance of :class:`Random`
may encounter contention and poor performance. Consider using separate instances of :class:`Random`
per thread instead.

[terryjreedy]

I agree. This replaces my comment.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[corona10]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@terryjreedy: please review the changes made to this pull request.