Random doc still talks about MersenneTwister instead of xoroshiro

[Seelengrab]

#40546 changed the default task-local RNG (as well as the default RNG) to Xoroshiro from Mersenne Twister, but the documentation still opens with an introduction to MersenneTwister. I think this needs to be adjusted before 1.7 is released, else the documentation is incorrect.