Document the usage for @CustomBinding

I think based on the current comments we have in https://github.com/Azure/azure-functions-java-library/blob/dev/src/main/java/com/microsoft/azure/functions/annotation/CustomBinding.java, we still need more information for both developers and tooling side, which may include:
- How to extend the `@CustomBinding` to generate a new annotation and samples
- Which properties are allowed to override and which are not allowed