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.

Scritto dal modello di indicizzazione a partire dal testo della issue.