Setting default region from credentials file

I just spent quite a while with a colleague trying to figure out why the aws shell wasn't honouring the default region that we'd set in `~/.aws/config` It turns out that somehow she had got a line in her `~/.aws/credentials` setting the default region to something else and this setting was being preferred. This seems like a bug, unless it is the intended behaviour that all files in `~/.aws` are valid locations for default settings?

Reproduce the issue with conflicting region values in ~/.aws/config and ~/.aws/credentials, then trace how the shell resolves the default region from those files. Confirm the intended precedence and document or test the behavior; done means the region selection matches that documented expectation.

Written by the indexing model from the issue text.