[RN][Codegen] Add docs for advanced iOS usage

[cipolleschi]

Update docs for Codegen for 0.76 and 0.77

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[coolsoftwaretyler]

Hey @cipolleschi - thanks for adding these docs. Such a cool feature of codegen.

I left a couple of typo fix suggestions for you, and I have one general suggestion. I learned about this field through this PR into react-native-cameraroll: https://github.com/react-native-cameraroll/react-native-cameraroll/pull/632/files#diff-7ae45ad102eab3b6d7e7896acd08c427a9b25b346470d7bc6507b6481575d519R122-R126

If you wanted to add examples of why or how to use these fields, I feel like that PR made a lot of sense to me (enable handling certain file handles that RCTImageURLLoader had dropped support for due to the lean core initiative.

Either way, I appreciate your time on this one. :)