Replace FIXME comment with explanation for JS safety in MutationObserver (#56271)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56271

Replace the FIXME(T157129303) comment in NativeMutationObserver with a
clarifying comment explaining why it is safe to call into JS at that
point: we only check mutations synchronously when committing from React,
so we are already in a JS task.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D98714071

fbshipit-source-id: 90738e9ec6e2215dff331a3b714373bf52f4b24b