Documentation on PRNG algorithms

[dhardy]

It would be good to update this documentation for the 0.6 release. Since documentation changes are not API breaking, it's not a blocker for 0.6, but especially with the work on #603 and #431 it is timely.

Points:

• the whole rand::prng module is deprecated, so the documentation should move (possibly to a document in the doc folder of the repo, possibly to new web page / book)
• ideally we should have a table giving an overview of many different algorithms with Rust implementations, not only those published by the Rand project