A follow-up to #368.

Likewise, it might be useful to add "calling a formatting function in a selector context" and "calling a matching function in a placeholder context" to the list of possible errors in formatting.md. Depending on how I read the "Unknown Function errors" section, maybe that's already implicit, but since users would probably find a "function called in wrong context" error more helpful than an "undefined function" error, it's probably better to be explicit.

Originally posted by @catamorphism in #368 (comment)