Add documentation for flipped mode

[unbelievableflavour]

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[unbelievableflavour]

Apparently there is already support for flipped mode (still gotta test this +add documentation for it).
For instance if you have a tileset like this

Then you should add a flipped mode like this:

[GuvBubbs]

So I tested this out in my test game

I took this...

Mirrored and flipped it to make this...

And what I found is that it places tiles in the wrong place now...

Take this example.

Notice how you can see the floor inbetween the wallpaper and the black wall top.

It should looks like so

If I turn the mirror setting off in LDtk this issue isn't seen.

If you're like me, your first thought would be that I've flipped things incorrectly. But if that were the case, it will offset mistake would be filled with white. Not transparency.