I am learning about DialogFlow and its integration with Google Assistant but I think it's a bit hard to develop because the users don't know all the posible topics that the chatbot can talk about. I know that this is probably a bad design from my side but I assume that there should be a "help" command to offer suggestions of the available Training phrases that a user can invoke, right?
This is not a build-in feature for Google Assistant (or any other integration as far as I know). Having a clear roadmap of available features/intents is often a challenge when deciding your chatbot's design. Here are some tips that might help you in this:
Build a custom help intent
With a custom help intent you would be able to assist your users in any way you see fit, you explain to them what your action is or offer them some suggestions. Since it is a custom intent you can really do whatever you want. As you asked about sending available training you could use the Dialogflow API to show them which training phrases are available in your bot to give them an example.
Use suggestion chips
This is probably the easiest option, when you user asks for help you can give them a set of standard suggestions to guide your user back on track. Your users can click on them or say what is in them to continue to a different intent. (Users that talk to your action on device without a screen can't see these, so you have to design an alternative for those devices too)
Example phrases in action overview
When publishing an action, you get the option to add some example phrases to get the user informed about what you action is designed to do. These suggestions only show up on the action overview so they don't help your users while interacting with your action, but it is still nice to add to help new users get started quickly.