The tooltip saying "either buffer full or profiler not started yet" could be clarified

[Here is an example](https://profiler.firefox.com/public/def4zbt1tyg8w0x7nkdtq8g7s6h3ad0ndz7jbzg/flame-graph/?globalTrackOrder=de0wc&hiddenGlobalTracks=12&hiddenLocalTracksByPid=29495-1w46~21926-0&localTrackOrderByPid=29495-780w6~5123-0~29556-0~526-0~29603-0~29692-01~5023-0~5053-01~29650-0~29726-0~4975-0~29481-0~21926-10&range=m261&thread=a&timelineType=cpu-category&v=6)

Either we already have the information to clarify this (the "starting profiler phase" could be known?), or we should add it from the backend.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-463)