Clarify decals usage

[Sleinmaster]

From Discord user skoy21_25673:

Hi to all. I recently stumbled on an issue with the Bandit Mask. I followed the instructions in the docs to add the DECALS_BANDIT_WINDOW_ALBD.PNG.DDS but in game, the mask appeared white. I found out that it needs to have the .FLAGS file in the textures folder for it to work. Can this be updated in the docs? It took me several days to figure this out.

That would be at https://docs.flybywiresim.com/dev-corner/texture-changes/ in the "Bandit / Racoon Mask" and "Elevator Trim Scale" section.