Add Gotchas to Documentation

[Ceyron]

• JAX allocates ~90% of the GPU memory by default. One has to close old JAX processes to start new ones. Often, this is also noticeable as CUDA being unable to obtain CuBlas support or in that some FFT planning routines fail
• The two forms of the nonlinear convection: Exponax now uses the "correct" nonconservative form, and APEBench kept the conservative form for legacy reasons. For a discussion, see here