Describe differences between REACT_METHOD and REACT_SYNC_METHOD

[asklar]

performance implications [](start = 127, length = 24)

I do not think it is a true statement. On their own the synchronous methods do not cause perf issues. In future TurboModules all methods are planned to be synchronous according to the FB team. Should we rather speak about the differences and limitations instead of on insisting using the REACT_METHOD?

Originally posted by @vmoroz in https://github.com/microsoft/react-native-windows-samples/pull/280#discussion_r498541384