enhance *src/keyreader.cpp* so it provides more value than xxd and od provides #2991
Description
Is src/keyreader.cpp used by anyone? I'm working my way through making the fish C++ code follow a consistent style and noticed the aforementioned module. It's not documented and doesn't do anything you can't do using other tools like od and xxd.
For 2.3.1 release:
- Documentation needs to match program output
- Fix HTML display error
- Update release notes