Add docstring to TestMacExcel test function.

macExcel_test.go

@@ -4,6 +4,10 @@ import (
 	"testing"
 )
 
+// Test that we can successfully read an XLSX file generated by
+// Microsoft Excel for Mac.  In particular this requires that we
+// respect the contents of workbook.xml.rels, which maps the sheet IDs
+// to their internal file names.
 func TestMacExcel(t *testing.T) {
 	xlsxFile, error := OpenFile("macExcelTest.xlsx")
 	if error != nil {