STEPS TO REPRODUCE
Open «System Settings» (systemsettings5) → Input Devices → Mouse → General → Choose «Double-click to open files and folders»
Start khelpcenter (in any way).
Choose menu item - "Kinfocenter Modules" - in the left half of the application window.
Choose "Memory" in the right half of the application window.
Choose «KinfoCenter» menu item in the left half of the application window.
Choose any submenu item of «KinfoCenter» menu in the left half of the application window.
Now the corresponding help is displayed in the right half of the application window.