Update auth documentation

**Problem**
When working with routes, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Start by locating the authentication route documentation and reviewing its current error-message guidance. Clarify which route errors need more descriptive messages and whether the requested change is documentation-only or also requires implementation; done means the agreed error cases and expected wording are documented clearly.

Written by the indexing model from the issue text.