UPDATED: Enter Description Tag for Selected v2.0
iTunes 12 no longer provides a means for editing the Description tag of multiple-selected tracks. This oldie, Enter Description Text for Selected, was just updated to v2.0 and will let you enter text for the Description tag of the selected tracks.
Technically (according to iTunes), the Description tag isn't supposed to be applicable to music tracks. But all track entries have an AppleScript description property.