util/winutil/authenticode: add missing docs for CertSubjectError

A #cleanup PR.

Signed-off-by: Aaron Klotz <aaron@tailscale.com>

util/winutil/authenticode/authenticode_windows.go

@@ -104,9 +104,12 @@ func queryPE(utf16Path *uint16, verify bool) (string, SigProvenance, error) {
 	}
 }
 
+// CertSubjectError is returned if a cert subject was successfully resolved but
+// there was a problem encountered during its extraction. The Subject is
+// provided for informational purposes but is not presumed to be accurate.
 type CertSubjectError struct {
-	Err     error
-	Subject string
+	Err     error  // The error that occurred while extracting the cert subject.
+	Subject string // The (possibly invalid) cert subject that was extracted.
 }
 
 func (e *CertSubjectError) Error() string {