Merge documentation for PostGenerationMethodCall.

This merges in changes provided by gotgenes to the previous
PostGeneration documentation to the new documentation provided by
rbarrois. This documentation relates to the new functionality of
overriding default arguments to declarations of
PostGenerationMethodCall.

Signed-off-by: Raphaël Barrois <raphael.barrois@polyconseil.fr>
Document need for mock/unittest.
Version bump to 1.3.0

Signed-off-by: Raphaël Barrois <raphael.barrois@polyconseil.fr>
doc: Fix rst.

Signed-off-by: Raphaël Barrois <raphael.barrois@polytechnique.org>