Add documentation for the class Document.

Little api refactor

src/xlsx/xlsxworksheet.h

@@ -77,6 +77,9 @@ public:
     bool loadFromXmlFile(QIODevice *device);
     bool loadFromXmlData(const QByteArray &data);
 
+    QString sheetName() const;
+    void setSheetName(const QString &sheetName);
+
     ~Worksheet();
 private:
     friend class Package;
@@ -84,8 +87,6 @@ private:
     Worksheet(const QString &sheetName, Workbook *book=0);
 
     virtual bool isChartsheet() const;
-    QString sheetName() const;
-    void setSheetName(const QString &sheetName);
     bool isHidden() const;
     bool isSelected() const;
     void setHidden(bool hidden);