Clarify the description of the `logo_mask` configuration key.

I had a hard time yesterday trying to understand why the logo I changed on my instance was displayed weirdly and some details of the picture were missing. I was browsing the same sections :fd and :frontend_configurations of dev/config.md at the same time and must have looked half a dozen times at logo_mask but failed to understand it could be linked to my «issue». I genuinely thought its effect was just to prevent the logo from being displayed at all so here's a proposal to make what it does a little bit clearer.