docs: document Disposable tsconfig lib requirement

[teamchong]

Summary

• document that explicit lib users need TypeScript's ESNext.Disposable component lib for Cap'n Web's Disposable references
• clarify that target has no Cap'n Web-specific minimum and only affects using emit
• clarify that DOM is shown as a browser example, while other runtimes should use their own environment types

Test

• npm run test:types

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commit: af96584