Document that the `auth` config section from the wildcard entry is not supported

[brandond]

• Ref: Wildcard Configuration in registries.yaml Partially Fails to Authenticate k3s#11352 (comment)

Wildcard configurations for both mirrors and configs should work consistently and allow pulling images when proper authentication is provided.
The documentation mentions wildcard support but does not specify limitations related to this case.

Relevant doc link: Private Registry - Wildcard Support

it is not intended that credentials would be looked up from the wildcard section.