Replace Field(description=...) with proper docstrings in auth models

From the V2 checklist in #1253 (under "Refactor the auth way of using it"):

Drop the use of Field(description=...), and use proper docstrings. As far as I know, we are not creating json schemas from it, so it's useless.

The auth-related Pydantic models currently use Field(description=...) to document fields. Since we are not generating JSON schemas from these models, the descriptions are not surfaced anywhere useful. They should be replaced with standard Python docstrings that are more discoverable via IDEs and help().

This is part of the broader auth refactor planned for V2.

Ref: Kludex's V2 checklist in #1253

Comienza localizando los modelos de Pydantic relacionados con auth que usan Field(description=...), y después revisa la lista de comprobación de V2 en #1253 para conocer el alcance previsto. Sustituye esas descripciones de campos por docstrings estándar de Python y verifica que ninguno de los modelos de auth incluidos siga usando este patrón de documentación.