[ci skip] Add documentation on interlock issue in Rails development mode

[wildmaples]

Description

Hello, it's my first time here! 👋

Closes #2352

The documentation is a new file docs/rails_dev_mode.md as suggested and pretty much a paraphrase/copy pasta of @earksiinni's analysis and observation.

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[cjlarose]

This is good to document. Thanks for your contribution! Just a few notes worth investigating:

• This problem isn't unique to puma. Any multithreaded web server you use to serve a Rails application will have the same problem.
• I haven't verified this, but this is only a problem with the "classic" autoloader. I suspect this isn't a problem with zeitwerk.
• Another workaround is probably to just remove the ActionDispatch::Reloader from the Rack middleware stack. That's the thing that prevents concurrent requests from autoloading.

[nateberkopec]

This workaround only works if these multiple processes are only using 1 thread. If you use cluster mode with multiple threads, the lock can still occur.