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.)

Das Issue nennt keine Quelldateien oder Tests. Beginne damit, die aktuelle ACT-IDL-Syntax für Methodenbeschreibungen zu prüfen, ermittle dann, wie Dokumentation im mehrzeiligen Element-Format dargestellt werden sollte, und verifiziere, dass die gewählte Form akzeptiert wird und den vollständigen Inhalt bewahrt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.