Improve `--help` output formatting

The current `--help` output format displays the usage information using `[]` and `<>` to indicate optional/required values as POSIX does, but the actual breakdowns in `ARGUMENTS` and `FLAGS` does not. It'd be nice if this was somewhat aligned so that this reads a little more clearly in the usage guidelines.

Start at the code that generates the `--help` output and compare the usage section with the `ARGUMENTS` and `FLAGS` breakdowns. The work is done when optional and required values use consistent `[]` and `<>` formatting throughout the help text.

Written by the indexing model from the issue text.