Add documents for class sqlite::backup

include/sqlite/backup.hpp

@@ -7,11 +7,37 @@
 struct sqlite3_backup;
 
 namespace sqlite {
+    /** \brief \a backup is a class for representing SQLite backup operations
+      * An object of this class is not copyable
+      */
     struct backup: boost::noncopyable {
+        /** \brief \a backup constructor
+          * \param conn_to takes a reference to the \a connection object
+          *        of the destination database
+          * \param conn_from takes a reference to the \a connection object
+          *        of the source database
+          */
         backup(connection& conn_to, connection& conn_from);
+
+        /** \brief \a backup destructor. The backup operation is automatically
+          *        finished after the object destructed.
+          */
         ~backup();
+
+
+        /** \brief Do a backup step
+          * \param nPage the number of pages to backup in this step. If a
+          *        negative integer is given, all pages are backuped. If this
+          *        parameter is omitted, -1 is used.
+          */
         bool step(int nPage = -1);
+
+        /** \brief Finish the backup operation
+          *        This is used for flushing current backup operation. After
+          *        this call, the backup object should not be used anymore.
+          */
         void finish();
+
     private:
         sqlite3* get_to_handle() const;
     private: