Allow documentation for **kwargs in wrappers OR turn off lint warning.

[aiuto]

Consider this wrapper method:

# buildifier: disable=function-docstring-args
def license(name, license_kinds = None, license_kind = None, copyright_notice = None, **kwargs):
    """Wrapper for license rule.

    Args:
      name: str target name.
      license_kinds: list(label) list of license_kind targets.
      license_kind: label a single license_kind. Only one of license_kind or license_kinds may
                    be specified
      copyright_notice: str Copyright notice associated with this package.
    """

First of all, the parsing must be broken, because (without the disable line) I see the lint warning

Argument "**kwargs" is not documented. 
 https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#function-docstring-args

I added this to the docstring
kwargs: additional args

And still get the above warning, plus

Argument "kwargs" is documented but doesn't exist in the function signature. 
  https://github.com/bazelbuild/buildtools/blob/master/WARNINGS.md#function-docstring-args

This is a no win.

Suggested solution: Allow this in the docstring
kwargs: [forwarded to <other rule name>]

The magical [fowarded to ...] would tell lint not to complain about it.
The could be used by Stardoc to pull up the definitions from the underlying rule, so we could document the wrapper appropriately.

[vladmos]

You should be able to document kwargs as **kwargs (with two asterisks), because just kwargs refers to a non-existing positional/keyword argument. In general, if the signature looks like

def f(foo, *bar, **baz):

you should define exactly the same literals in the documentation:

Args:
  foo: ...
  *bar: ...
  **baz: ...

I don't see whether the magical [fowarded to ...] is useful, Buildifier doesn't analyze the documentation of each argument, it just makes sure it exists.