Improve some hint texts

Description:

Suggestion: Sentence text and intent cannot be blank.

Suggestion: Highlight a word to add an entity. An entity is a relevant piece of information from a user’s message, such as dates and addresses.

Suggestion: Intents are used to describe how user messages should be categorized.

Browser: browser
OS: operating system
Authenticated: Yes/No
URL: https://staging.bothub.it/peter_miguel/susana/training