Update the reproducer message (#50848)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50848

This clarifies our reproducer guidelines.
It's also currently pending on this other PR on the website:
https://github.com/facebook/react-native-website/pull/4589

Changelog:
[Internal] [Changed] -

Reviewed By: cipolleschi

Differential Revision: D73194317

fbshipit-source-id: 35a0e6b4def734c8cff5099ab426d024f9ff6c4d