Merge branch 'arkaprabha/add-documentation-for-incident-severity-change' into 'master'

Add documentation for incident_timeline_events_for_severity flag

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/102024



Merged-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Approved-by: Marcin Sedlak-Jakubowski <msedlakjakubowski@gitlab.com>
Co-authored-by: ArkaprabhaChakraborty <chakrabortyarkaprabha998@gmail.com>