Document wxDateTime::GetValue()

It was referred to in GetTicks() description but not documented itself, add it
now.

interface/wx/datetime.h

@@ -566,6 +566,15 @@ public:
     */
     time_t GetTicks() const;
 
+    /**
+        Returns the number of milliseconds since Jan 1, 1970 UTC.
+
+        Directly returns the internal representation of wxDateTime object as
+        the number of milliseconds (positive or negative) since the Unix/C
+        epoch.
+     */
+    wxLongLong GetValue() const;
+
     /**
         Returns broken down representation of the date and time.
     */