Clarify placeholder usage (‘foo’/‘bar’) in CLI flag documentation

[SuperHeavyBallet]

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

Location Example: https://github.com/npm/documentation/blob/main/content/cli/v11/using-npm/config.mdx