Merge branch '415339-docs-instance-headers-destroy-1' into 'master'

Added documentation for instance headers destroy API

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124409



Merged-by: Fiona Neill <fneill@gitlab.com>
Approved-by: Fiona Neill <fneill@gitlab.com>
Reviewed-by: Fiona Neill <fneill@gitlab.com>
Co-authored-by: Hitesh Raghuvanshi <hraghuvanshi@gitlab.com>