chore: remove references to StyleSheetRegistry from StyleSheet docs

[alecmarcus]

Based on some digging, it seems as though StyleSheet no longer uses a registry of IDs:

react-native-community/discussions-and-proposals#400
#629 (comment)

This PR updates the docs to reflect that change.

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[javache]

This is absolutely true, but we do want to keep the option of optimizing this data structure in the future. Users should also not assume a style object is something they can mutate or introspect (without going through flatten), so it'd be great to acknowledge that in the docs.

[rikur]

Now I'm even more confused. The StyleSheet could really use an update so that users know the best practices.

I've been living in a make-believe world of StyleSheet IDs moving through the bridge for years 😕