documentation: add brief description of general features of models

I think we should add a new section with a brief descriptions features that are the same across models.

The case I have in mind is the description of the new nan/missing value handling.
It's described in the docstrings (I guess), but I would like to add a section BEWARE to it.
The advantages of cleaning first: You need to do it only once, if the same model is estimated repeatedly (no repeated checking and array copying). When we run regression with different number of exog variables/columns, then the number of observations can differ after removing missing observations in each exog. This is good in some cases, but not when we want to compare (nested) models.

other features ?

• pickle for prediction :?
• constant handling : cleaned up in 0.5
• t versus normal distribution in tests/postestimation inference

I don't remember if or where those are already described.

Revisa los docstrings de los modelos existentes y la documentación sobre el manejo de nan/valores faltantes, el manejo de constantes, la predicción basada en pickle y las distribuciones t frente a las normales en las pruebas y en la inferencia posterior a la estimación. Se considera completado al añadir una sección de características generales claramente delimitada, incluida la advertencia solicitada sobre valores faltantes y cualquier característica entre modelos que se confirme.