Correction to Users API documentation

What does this MR do?

Currently the API documentation for User creation says that the user name is not a required attribute, but it is required according to the code. This corrects the documentation.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content