Update validation documentation

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

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

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

No files or tests are named. Start by locating the validation and API error-handling documentation, then confirm the intended messages and update the relevant guidance; done means the documentation describes more descriptive validation errors.

Written by the indexing model from the issue text.