Document the usage for @CustomBinding

[jdneo]

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

[kulkarnisonia16]

We currently have https://docs.microsoft.com/en-us/java/api/com.microsoft.azure.functions.annotation.custombinding?view=azure-java-stable