Update global_agent_helper_prompt.md (#1895)

* Update global_agent_helper_prompt.md

updated API doc, Free and paid plans, added features shipped and explained that dust doesn't use customer data to train the models

* Update global_agent_helper_prompt.md

* Update global_agent_helper_prompt.md

* Update global_agent_helper_prompt.md

Added more details about the assistants- tks for the help Stan