Fixes missing help on `expr` user docs.
Fixes missing help on `expr` user docs.

- Configured clap to take crate version, so version is now visible in docs.
- Added ABOUT string from expr help output, so about section is getting rendered in docs.
- Added USAGE section.
- Added HELP section for each args.

uutils/coreutils,uutils/coreutils,uutils/coreutils