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

Start with src/main/java/com/microsoft/azure/functions/annotation/CustomBinding.java and review its current comments. Document how developers can extend @CustomBinding to create a new annotation and samples, and explain which properties tooling may or may not override. The work is done when both audiences and both requested topics are covered.

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