Explain the role and rules for `where` row conditions.

where (, any and all) uses the Expr::RowCondition internally, we should probably have a small section on what rules are valid for those. E.g. mention the $it special variable (#401) and that columns of tables can be accessed by their bare name (or the quoted string of their name if they are not valid variable names). Then we can refer to those from the command documentation (@hustcer is there a good way we could allow commands to link to pages in the book?)

Localiza las páginas del libro o de la documentación del comando para where y lee las referencias a Expr::RowCondition y issue #401. Añade una pequeña sección que describa reglas válidas para las condiciones de fila, incluidos $it y nombres de columnas sin comillas o entre comillas, y enlázala desde la documentación del comando correspondiente si el libro lo admite.