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.)

The issue names no source files or tests. Start by reviewing the current ACT IDL syntax for method descriptions, then determine how multiline element-form documentation should be represented and verify that the chosen form is accepted and preserves the full content.

Written by the indexing model from the issue text.