Add external and exports to lib mode example

Editor assessment

A perfectly serviceable documentation commit. The docs type is correct, the subject is imperative, lowercase, and specific enough — readers know the lib mode example gains external and exports configuration — and the length is comfortable. No scope: (docs) or (guide) would have been a minor bonus for filtering, but plain docs is entirely conventional. No body or issue reference: for an example expansion this is acceptable, though a sentence on why the example needed those fields (likely demonstrating proper externalization in library builds) would help. There is nothing misleading here, no punctuation problems, and docs changes correctly produce no version bump under most release tooling. It is unremarkable in the best sense — does its job and gets out of the way.