Document array values for tsconfig extends

[StaryMoon]

Summary

• Update the tsconfig extends option docs to describe string arrays
• Clarify that later entries in the array override earlier entries
• Add a short example using multiple base configs

Related issue

• Document that compiler.extends can be an array TypeScript#62915

Testing

• Ran git diff --check
• Documentation-only change; site build not run because Node.js / npm / pnpm are unavailable in this environment