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.)

この issue ではソースファイルもテストも指定されていません。まず、メソッドの説明に関する現在の ACT IDL 構文を確認し、次に複数行の要素形式のドキュメントをどのように表現すべきかを判断して、選択した形式が受け入れられ、内容全体が保持されることを検証してください。

索引モデルが issue の本文から書いたものです。