docs: clarify os/cpu/libc accept string or array

[SatvikMishra08]

Summary

Clarifies that os, cpu, and libc in package.json each accept a string or an array of strings, matching npm's runtime behavior in npm-install-checks (checkList coerces a string to a one-element array).

Previously the libc example used bare strings while os/cpu showed only arrays, with no note that both shapes are valid — which has confused authors and downstream tools (see #10002).

Changes:

• Document string-or-array for all three fields
• Show both array and string examples side-by-side
• Align the libc section examples with os/cpu (array form + string shorthand)

Verified against node_modules/npm-install-checks/lib/index.js checkList on current latest. No maintainer comments on the still-Needs-Triage issue.

Fixes #10002

Test plan

• Docs page sections for os / cpu / libc read consistently
• Examples are valid JSON and match runtime acceptance of both shapes