Document dependency between streams in PCG

[vigna]

PCG streams obtained by modifying the increment constant are not independent. The topic has been discussed at length for the Rust implementation:

rust-random/rand#907

This should be something the user is made aware of, as having a parameter and talking about "streams" in the documentation gives the idea that the streams are somehow independent.

[bashtage]

@vigna I have substantially weakened the language and removed references to "independent" pretty much everywhere, using distinct in place. Thanks for the feedback.