This can get very subtle, but both a function (a.k.a. a table of methods), and a method (a specific implementation of the function) have a docstring. The function docstring is usually concatenated from the docstrings of the methods, but it is both common and useful to add a single docstring either to the empty function object or to the first / most general method. That docstrings then serves as the docstring for the entire function and may cover multiple methods. In that context, the function docstring would specify one or more call signatures, which often can’t be derived by introspection.
As an aside, within Documenter, the @docs block allows you to specify whether you want to show a function docstring or a method docstring.