Missing docs for ObjCClass/ObjCMethod/ObjC decorators

[PeterStaev]

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[ ] A typo [ ] Wrong documentation [ ] Improvement of existing article [x] Missing documentation [ ] New article needed

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In the old docs there were examples showing the easier way to make classes use protocols or to export methods to Objective-C in this section https://docs.nativescript.org/core-concepts/ios-runtime/how-to/ObjC-Subclassing. This is missing from the new docs. You can read more about it in https://github.com/NativeScript/ios-runtime/pull/753 . There is a link to a PR in the old io-runtime-docs repo so hopefully the info can easily be added back