LibreOfficeKit: add doxygen since tag to _LibreOfficeKitClass struct

So it's easier for people adding language bindings know the
libreoffice version they depend on.

Change-Id: Ibe4526156663bb1250558b3f0e8517747a8add66
Reviewed-on: https://gerrit.libreoffice.org/21873Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Riccardo Magliocchetti <riccardo.magliocchetti@gmail.com>

include/LibreOfficeKit/LibreOfficeKit.h

@@ -53,9 +53,11 @@ struct _LibreOfficeKitClass
 
     char* (*getError) (LibreOfficeKit* pThis);
 
+    /// @since LibreOffice 5.0
     LibreOfficeKitDocument* (*documentLoadWithOptions) (LibreOfficeKit* pThis,
                                                         const char* pURL,
                                                         const char* pOptions);
+    /// @since LibreOffice 5.2
     void (*freeError) (char* pFree);
 
 #if defined LOK_USE_UNSTABLE_API || defined LIBO_INTERNAL_ONLY