Enable AJAX lang string requires amending

Following on from MDL-32908 the lang string configenableajax/admin requires amending:

This setting allows you to control the use of AJAX (advanced client/server interfaces using Javascript) across the whole site. With this setting enabled users can still make a choice in their profile, otherwise AJAX is disabled for everybody.

I suggest something like:

This setting controls the use of AJAX across the site. AJAX is required for certain functionality such as drag and drop.