wrap roxygen template for YAML source

[malcolmbarrett]

In several functions, I have documentation that looks like this:

This field is not available in all output formats; it is available in pdf_document(), html_document(), word_document(), odt_document(), and powerpoint_presentation() by default.

It may be good to refactor this as a roxygen template function eventually to make editing this easier. It's also possible a registry system (as in #16) could be used to generate this automatically