Multiline API documentation

Documentation for elements in ACT IDL is currently specified like so:

```XML

```

Having the description as an attribute makes perfect sense, but when writing an IDL file it makes it very hard to provide comprehensive documentation.

It would be nice, for example, to be able to provide documentation in element form also:

```XML

An extremely useful method.

This is an implementation of Fred Frobricator's notion of _Frobrication_, introduced in
the 1867 paper 'On Frobrication', of which you can find a copy here:

https://www.en.wikipedia.org/wiki/Frobrication

```

(Also happy with any other solution that allows for documentation in a similar for to my example, be it a different syntax for specifying an attribute of which I am unaware, special comments, or whatever.)

El issue no menciona archivos fuente ni pruebas. Empieza revisando la sintaxis actual de ACT IDL para las descripciones de métodos; después, determina cómo debe representarse la documentación en formato de elemento multilínea y verifica que la forma elegida se acepte y preserve todo el contenido.

Escrito por el modelo de indexación a partir del texto del issue.