Issue Description:

The documentation currently states:

To make a custom type conform to the RandomNumberGenerator protocol, implement the required next() method. Each call to next() must produce a uniform and independent random value.

This is completely correct and fully specifies the behavior, but we should probably be a little bit more clear that in order to function correctly, it must generate all possible `UInt64` values (or equivalently that each bit is uniformly 0 or 1). Ideally we should give some examples of things that are not acceptable implementations (e.g. calling `rand()` or `lrand48()` and converting the result to `UInt64`).

Spun off from SR-11447