Make a method structure reflect its docstring

This way the method is easier to read once you've read the docstring.

Tests:
Raise a specific error when args cannot be associated to a type

Using a specific error will make it clearer where the error comes from and
what it is.

Tests:
Add a test for missing docstring for arguments

Make sure we get an empty string if an argument is missing its docsting

Tests:
python parse_this_test.py OK