Summary:
Trivial PR. Cleaning up the CODEOWNERS file a bit.

[GENERAL] [Changed] - Clean up and annotate CODEOWNERS on GitHub
Pull Request resolved: https://github.com/facebook/react-native/pull/23678

Differential Revision: D14253726

Pulled By: cpojer

fbshipit-source-id: ffb47c18b7078b7489dadb352cdaa5ffea0d0aea