Improve std os documentation #2

Improving documentation, consistency, removes evils empty lines etc...

I think that @huonw pointed out earlier, but the `let key = ...` and `let value = ...` seems a little odd, could you use constant strings instead?

Additionally, can you place more emphasis on `setenv` like this:

```rust
use std::os;

// Override the previous value for `KEY`
os::setenv("KEY", "VALUE");
```