Users can now add a comment when dismissing code scanning alerts

Users can now add a comment when dismissing a code scanning alert.

It is optional to provide a dismissal comment. Dismissal comments are recorded in the alert timeline. They can also be set via the code scanning REST API when updating an alert, and retrieved through the new dismissed_comment attribute.

This feature is now available to all users on GitHub.com and will be released in GHES 3.6.