In ProseMirror editor, document util.walkUpUntil()

[toolness]

I was writing some code today and decided to use util.walkUpUntil() but it wasn't immediately obvious what it'd return if it didn't find a node that matched the predicate. After reading the code, I intuited that it was -1, but it'd be nice to document.

Also, it'd be nice to document the return type in the function signature too!