Update documentation for Hlt2 line monitors

• Rewrite text and add more structure to make the docs hopefully clearer.
• Remove use of default_monitoring argument, which has been removed some time ago.

added Monitoring only GitLab CI labels

assigned to @jagoodin

Hi @jagoodin! This can most likely go in directly after your review if the gitlab pipeline passes, as it only touches the documentation.

added RTA label

resolved all threads

assigned to @rmatev

mentioned in issue #669 (closed)

assigned to @mramospe and unassigned @jagoodin

mentioned in issue #671 (closed)

assigned to @bjashal and unassigned @mramospe

approved this merge request

merged

mentioned in commit 000a72c6