Comments can be made to the build at any time. Navigate to the Test Coverage primary tab and then to the Builds secondary tab. In the specific build's details window, click on the comments icon in the right side vertical panel. Enter your comments in the box and click Post Comment to submit. If there is a purpled numbered box below the comments icon, that indicates the number of comments already made.
Users will need view permissions in the test coverage module to add a comment to the build. Depending on the settings of a users project permissions, users may be able to view all project builds or just a subset of them. Project owners manage the permissions for project members.