Doc: Add binding for win.dual-odd-left to help-overlay

Hi,

This is a follow-up to !378 (merged); it adds the new binding for win.dual-odd-left to help-overlay.

I've tried to wrap my ahead around the translations with the .po files, but I've run out of time. I've tried looking for pointers in the Makefile, but I didn't find anything concrete.

I've still added the French translation in 6091fc05, but please bear in mind that I've just edited the file by hand, and that it is unlikely to work as is.