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.)

La issue non indica alcun file sorgente né alcun test. Inizia esaminando la sintassi attuale di ACT IDL per le descrizioni dei metodi, quindi determina come rappresentare la documentazione in formato elemento multilinea e verifica che la forma scelta venga accettata e preservi l’intero contenuto.

Scritto dal modello di indicizzazione a partire dal testo della issue.