Clarify the reference to aws:region in the namespaced-config docs

[cnunciato]

@thomas11 pointed out that in the docs for accessing namespaced configuration, we mention how to do the lookup, but don't make it clear where the setting is that's being looked up.

This is made more confusing when the selected language is YAML because the program file contains a config section, but no aws:region setting:

We should clarify that these snippets all assume the existence of a stack-config file (Pulumi.dev.yaml for example) that looks something like this:

config:
  aws:region: us-west-2