Rework comparisons in the docs of JAX vs. NumPy PRNG

[emilyfertig]

Parts of the docs (https://jax.readthedocs.io/en/latest/key-concepts.html#pseudorandom-numbers, https://jax.readthedocs.io/en/latest/random-numbers.html) imply that NumPy PRNG needs global state. NumPy currently recommends passing around an (implicitly updated) local PRNG state, and we should update our docs so the comparison is more fair. (see #24917 (comment)).

[jakevdp]

I found the original issue: #11026