[Libreoffice] Decoview: improve comment about checkmark mirroring

[Matteo Casalin]

This patch improve the comments related to checkmark mirroring: it seems 
to be needed to compensate the lower level mirroring performed by a 
device with mirrored graphics.
Please verify that I understood correctly the code (can graphics really 
be mirrored at a lower level?) and take into account that, according to 
RTL people [1], checkmarks are drawn the same in LTR/RTL languages.

Matteo

[1] Lior Kaplan and Khaled Hosny in the thread "Remember the RTL 
interface", dated 11/29/2011.