SchemaShell --name and --file params priority fix.

Corrects the two issues identified by #4174 and CakeDC/migrations#183.

I don't currently have the proper test environment set up to run the core test suite locally, so I'm going to let Travis check things for me. I wouldn't be surprised if there is a test to fix somewhere.
To be honest, I followed the popular convention set by most of _existing_ doc blocks in the file so that my additions wouldn't "stand out," but I will update as requested since I agree with the principle.