build: update coverage Makefile target comments

Update the comments for the `coverage` Makefile target.
- Source files under `lib` are no longer instrumented since
https://github.com/nodejs/node/commit/d1dee495db1451e74f5364c8687148a53ff0b50d
- Fix the name of the related CI job.

PR-URL: https://github.com/nodejs/node/pull/39365
Refs: https://github.com/nodejs/node/pull/25157


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>