Improve std os documentation #2

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

This example seems to be emphasizing `getenv` more than `setenv`, perhaps more attention could be drawn to the call to `setenv`? 

Additionally, it's stylistically more acceptable to invoke functions through one layer of modules, `os::setenv` rather than two, `std::os::setenv`. Could you import `std::os` at the top?