Recommended text for seed documentation

[xpe]

Recommendation

I'd suggest adding some more context to:
https://docs.rs/rand/0.4.2/rand/trait.SeedableRng.html

Currently, it says:

Create a new RNG with the given seed.

Example 
...

I'd suggest adding text so that it looks more like this:

Create a new RNG with the given seed.

Note: the seed varies based on the chosen generator. Most RNG's 
expect `&'a [u32]` values but some expect `&'a [u64]`. Please refer to
the particular RNG documentation for the length of the seed.

Example 
...

Motivation

When I first looked into how seeds worked in this library, I tried to find a definition of Seed to get some clarity. That didn't help much, since Seed is a generic for the SeedableRng trait.

pub trait SeedableRng<Seed>: Rng {
    fn reseed(&mut self, Seed);
    fn from_seed(seed: Seed) -> Self;
}

Particularly for people new to the library, saying a few words about what a Seed is may help.

[dhardy]

Take a look at dhardy#18 if you want to know what plans are in store; this trait is being heavily revised (part of that is restrictions on Seed).