Document `Xoshiro(seed)`

[Seelengrab]

The default docs for Xoshiro only document the type, but not the seeded creation method like for MersenneTwister:

help?> Xoshiro
search: Xoshiro

  Xoshiro

  Xoshiro256++ is a fast pseudorandom number generator originally
  developed by Sebastian Vigna. Reference implementation is
  available at http://prng.di.unimi.it
[...]

help?> MersenneTwister
search: MersenneTwister

  MersenneTwister(seed)
  MersenneTwister()

  Create a MersenneTwister RNG object. [...]