docs: more detail for command line flags

[SuperHeavyBallet]

The documentation for command-line flags uses “foo” and “bar” as placeholders without clarifying that they are generic examples of a key and a value.

New readers unfamiliar with this convention may interpret them as literal parameters. Suggest adding a short note as given explaining that “foo/bar” are placeholders for any config key/value.

This branch includes a full rewrite.

This change was originally suggested in npm/documentation#1760 — applying it here since the CLI docs are the true source