Official API

When you select a function from the drop-down list on the editor toolbar, help for that function is displayed:

When you move the cursor off the Script Editor Help pane, the pane is hidden ("unpinned" or "rolled up"). You can still see the title of the window pane in a tab positioned at the right-hand edge of the script editor (where Help pane was docked):

To temporarily display the Help pane click the tab. While the pane is displayed, you can click the pin icon to "pin" it (i.e. turn the Auto-hide property off and display it permanently):