Improve std os documentation #2

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

Also, this doc block is sadly not being applied for windows. Could you restructure to something like this:

```rust
/// dox
pub fn setenv(k: &str, v: &str) {
    #[cfg(unix)]
    fn real_setenv(k: &str, v: &str) { /* ... */ }

    #[cfg(windows)]
    fn real_setenv(k: &str, v: &str) { /* ... */ }

    real_setenv(k, v);
}
```