PRNG module documentation #622

Thanks largely to @pitdicker we have quite a bit of documentation on PRNGs in the PRNG module.

Unfortunately, it can't stay there (long). All items in that module are now deprecated (although the module itself is not).

In theory the small-rngs repo can house documentation on small non-crypto PRNGs. But what about the other PRNGs, and what about "general" documentation?

We do have a doc subdir which could house some of this documentation, although links to this from the Rustdoc will be needed.