Document API endpoit usage

[rodmaia]

Some endpoints have a more complex usage pattern. Utters, for example, can have one or more content alternatives, and each alternative can have multiple strings (balloons).

This type of usage needs to be clearly documented so new contributors know how to explore the full potential of the API.

See also #42

Tasks

• [ ] Consider naming utterText to something more clear, like utterTextAlternative
• [ ] Steps to complete this task

Acceptance criteria

• [ ] Developers outside the Botflow team know how to use the API just by reading the documentation

[mansogf]

Following the procedures according to #153.