DOC: Do not use types for *args, **kwargs (#585)

Docstrings are primarily intended for the caller, and there *args, **kwargs are not variables with a type but a signature pattern (this is also hinted at by using *args, **kwargs instead of args, kwargs as parameter names). It's rather confusing to try and type them, I'd even claim it's misleading and will tempt less experienced programmers to call `def func(*args)` with *args declared as tuple with a single tuple argument `func((1, 2, 3))`.

This omission is also not an information loss for the few people, who look at the signature from the function body perspective (aka developers). That args and kwargs resolve to a tuple / dict in the function is a hard-coded fact.

doc/format.rst

@@ -225,11 +225,11 @@ description, they can be combined::
       Input arrays, description of `x1`, `x2`.
 
 When documenting variable length positional, or keyword arguments, leave the
-leading star(s) in front of the name::
+leading star(s) in front of the name and do not specify a type::
 
-  *args : tuple
+  *args
       Additional arguments should be passed as keyword arguments
-  **kwargs : dict, optional
+  **kwargs
       Extra arguments to `metric`: refer to each metric documentation for a
       list of all possible arguments.