Fixes #357 - Hide subscriptions.validate and various documentation fixes

What does this MR do?

• Moved the idea for validating subscriptions to the debugging namespace. It's still used in index.js from its proper place in subscriptions but in terms of API access we can keep using it in our tests. This means it's no longer exposed through the API. I've renamed the function from validate to validateSubscriptions to make the intent a bit more clear.
• The rest of the changes are all fairly transparent based on the list in the issue.

Integration notes

The validate function in the subscriptions namespace is not available anymore.