Add a note in the docs that private functions are not allowed as hooks

[vkarak]

We should also add a tip that if you are writing a library it's a wise idea to properly make the names unique by following a pattern _<ClassName>_<HookName>. This will avoid conflicts with hooks defined in subclasses which can go unnoticed. Also, with the introduction of the always_last argument, there is no need for exposing the hooks and overriding them.