unref event is not documented

Perhaps this is deliberate, but the `'unref'` event fired at https://github.com/pouchdb/pouchdb/blob/c8e532e4d817aa6d6122f7944967c0293c3a26a9/packages/node_modules/pouchdb-core/src/constructor.js#L28 and tested at https://github.com/pouchdb/pouchdb/blob/c8e532e4d817aa6d6122f7944967c0293c3a26a9/tests/integration/test.close.js#L87-L119 is not documented.

Start with the `unref` event emission in `packages/node_modules/pouchdb-core/src/constructor.js` and its coverage in `tests/integration/test.close.js` lines 87-119. Find the relevant PouchDB API or event documentation, document when the event fires and what it represents, and confirm the documentation matches the tested behavior.

Written by the indexing model from the issue text.