[Random] Document default_rng and remove references to GLOBAL_RNG

[giordano]

Following discussion on Slack. Note: there are still a couple of references to GLOBAL_RNG in the docstring of seed!

julia/stdlib/Random/src/Random.jl

Lines 386 to 387 in 7189c81

	seed!([rng=GLOBAL_RNG], seed) -> rng
	seed!([rng=GLOBAL_RNG]) -> rng

because as far I can tell that's what's still actually used:

julia/stdlib/Random/src/RNGs.jl

Lines 375 to 376 in 7189c81

	seed!(seed::Union{Nothing,Integer,Vector{UInt32},Vector{UInt64}}=nothing) =
	seed!(GLOBAL_RNG, seed)

CC: @cossio

[rfourquet]

I think you should also make the change for seed!, the only difference is that GLOBAL_SEED is mutated, but that's an implementation detail.

[giordano]

Done, thanks! Now there shouldn't be any more references to GLOBAL_RNG in the documentation.