Manual:$wgNamespacesToBeSearchedHelp

Details
Namespaces to be searched when user clicks the "Help" tab on Special:Search with the new search interface.

Same format as $wgNamespacesToBeSearchedDefault.

To have a working search header you have to take care of a few things:
 * Namespaces you set in  should not be set in.
 * The order of the set namespaces in the array is important! Be sure to set them in the order of the namespace number. So e.g. not Help before Project.