catalog.rst: Add __iter__ to Catalog documentation
catalog.rst: Add __iter__ to Catalog documentation

The declaration of __iter__ under the special-members
directive makes it visible in the documentation.
The docstring describing __iter__ already exists.

Closes https://github.com/python-babel/babel/issues/128

mitsuhiko/babel,python-babel/babel,mitsuhiko/babel,mitsuhiko/babel,python-babel/babel