Improve documentation for rand::random

This is now linked to in the guide, so I want to make sure it's good. This
adds a bit more explanation, and brings usage in line with current good style.
auto merge of #15746 : steveklabnik/rust/docs_random, r=alexcrichton

This is now linked to in the guide, so I want to make sure it's good. This
adds a bit more explanation, and brings usage in line with current good style.