Informative function signatures, allowing programmers to add per-function documentation, and tooling that displays those things are by no means limited to staticly typed languages.

I guess knowing allowed types for each parameter is informative (more if very specific types are used rather than string or integer), but documentation usually specifies that anyway. And even knowing all the types isn't usually enough without documentation.

My point is just that I've used plenty of libraries in dynamically typed languages without needing to read the source. And conversely, I've occasionally needed to look at the source of a function in a staticly typed language, to answer some question not answered by the types and documentation.