Python: Document (parts of) `ExternalArtifact.qll`.
Python: Document (parts of) `ExternalArtifact.qll`.

I don't think there's any need to document the parts specific to
metrics or defects, as I don't believe these are used anywhere.

github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql