Python bindings: use cmake_parse_arguments, fix documentation

Summary:
Use of positional arguments is error prone, as e.g. switching order
of NAMESPACE and MODULENAME wont have the desired effect, as
the keywords are ignored for the first three arguments.

Parse all arguments with cmake_parse_arguments, and fix the documentation
(list supported arguments) and example (add TARGET keyword).

Test Plan: build bindings

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D15559