FR: Documentation of timeline feature

Please forgive the newbie question, but I can't seem to find any documentation which describes the meaning of these blue rectangles below the CPU usage graph:


I'm sure that the matrix room would be able to help me out, but it would also be nice if this part of the timeline were also explained [in the docs](https://profiler.firefox.com/docs/#/./guide-ui-tour-timeline)

Thanks!

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-842)