Specification for Scheme/LilyPond function documentation

[uliska]

We need a specification how to document public-facing functions in LilyPond and Scheme files. From this doc-comments regular documentation has to be generated later, ideally one would be able to use existing tools to do that.

The documentation has to provide the following information (to be discussed and completed):

• function name
• return type
• argument names/types (with description)
• general function description
• more?

Blocks #26