Make it clear which functions are formatting functions and which are selection functions

[stasm]

@markusicu wrote in #230 (comment):

The syntax so far is not clear about formatting functions and selection functions. It looks like "number" could be both, but they should be separate. "number" should just format, and "plural" should both format and select (remember that the two are intertwined).

[aphillips]

This is addressed by recent design documents and is implemented in the registry parts of the spec. Open new specific issues if needed.