Add examples for Rng functions.
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auto merge of #5825 : danluu/rust/rngdoc, r=erickt

This adds an example for most of the methods in Rng.

As a total newcomer to Rust, it took a while to figure out how to do basic things like use library functions, because there aren't many usage examples, and most examples that Google turns up are out of date. Something like this would have saved me a bit of time.

This might be a bit verbose. Some alternative options would be to consolidate all the examples into one section, or to only have code for the specific function call inline.