Possible documentation clarification needed for nalgebra::zero

As a newcomer to nalgebra, I was a bit confused about the various `zero()` and `zeros()` functions for constructing matrices.

I was originally disappointed that I would need to repeat the dimensions for each construction like
```
#[derive(Debug)]
struct S {
x:MatrixMN,
}
fn main() {
let s = S { x:MatrixMN::::zeros() };
println!("{:?}", s);
}
```

I eventually came across `http://nalgebra.org/rustdoc/nalgebra/fn.zero.html`, which allows me to just write
```
let s = S { x:zero() };
```

I'm still not entirely clear on the reasons for both `zero()` and `zeros`, so maybe an explanation could be added to the otherwise excellent documentation?