Update doc for k8s.io/code-generator/cmd/conversion-gen
Update doc for k8s.io/code-generator/cmd/conversion-gen

Added explanation of the `k8s:conversion-gen-external-types` comment
tag.

Added explanation of how the developer can selectively override the
generated conversion functions.

Also updated description in Makefile.generated_files.

Kubernetes-commit: 4982cbb2d8ab4330a871c59567e81269a96d1312

kubernetes/code-generator,kubernetes/code-generator