Users of languages can't be expected to. Therefore the language designers and maintainers themselves, especially if they're working on the stdlib, should do so, IMO. It's not only education for a proficient programmer, it helps to understand the underlying system you're building on and it's security assumptions.

The random char device code isn't that hard to understand, and if you're not a strong C programmer (the Ruby-core people are good C programmers, I suppose) - there's a paper explaining how it works: https://eprint.iacr.org/2012/251.pdf

Aaron