CWG2667 [cpp.import] Example should demonstrate that module imports do not import macros

[zygoloid]

Reference (section label): [cpp.import]

Link to reflector thread (if any): https://twitter.com/starfreakclone/status/1603571154245275653

Issue description:

We have an example in [cpp.import] of header unit importation adding points of definition for macros, but the example does not make it clear that named module imports do not have that effect.

Suggested resolution:

Add a module unit to the example in [cpp.import]/8 that imports the header unit, and explain that importing that module unit does not make macros from the header unit visible.

[jensmaurer]

CWG2667