Docs: Improve documentation for `Custom_Image_Header` and `Custom_Bac… #25850

Job	Run time
Check PHP compatibility	18s
Slack Notifications / Prepare notifications	4s
Slack Notifications / Success notifications	6s
Slack Notifications / Failure notifications	0s
Slack Notifications / Cancelled notifications	0s
Slack Notifications / Fixed notifications	0s
Failed workflow tasks	0s
	28s

Provide feedback