Clarify in documentation how modules are determined to be of a certain type

[johnthagen]

From the Limitations section of the docs:

The classification of an import as being non-stdlib of some kind depends on that package actually being installed.

But looking at the code in _import_type() it seems like what really happens is that if the module doesn't match the others, it's considered to be third party. This was confusing at first because based on the documentation I tried installing/uninstalling third party packages to try to fix issues:

        else:
            # Not future, stdlib or an application import.
            # Must be 3rd party.
            return IMPORT_3RD_PARTY

Next:

You will want to set the application-import-names option to a comma separated list of names that should be considered local to your application. These will be used to help categorise your import statements into the correct groups.

Also from _import_type(), it would be helpful to new users to mention that relative imports are always considered local:

        if name is None:
            # relative import
            return IMPORT_APP
...
        elif isinstance(node, ast.ImportFrom) and node.level > 0:
            return IMPORT_APP_RELATIVE

I realize these are somewhat implementation details, but I think they really help new comers determine why many of the locals are properly recognized, but not others.