pitfall: 2d camera and sprites not showing properly

[inodentry]

Clarify that bevy's 2d camera bundle constructor creates a Transform that positions the camera at Z=999 (or something like that, close to the clipping plane). For 2d to behave as expected (Y axis up, looking towards -Z, background at Z=0, higher sprite layers at positive Z coorditnates), this must be preserved.

If users try to create their own transform for their 2d camera, and don't put it at this far-away Z coordinate, they might wonder why they can't see their sprites, or why sprites appear flipped, or why they need to use negative Z coordinates, etc...