Nothing in the manual (at least not knowing the name of the thing). Nothing when using the mouse (function name or shortcut). Nothing under the heading: keyboard editor (at least in the French help). Even the video tutorial on the keyboard editor goes over the description of the tools.
Here is the text, for those who are also wondering:
Sync region tool (MIDI CC and dynamics editor only). When you are viewing multiple tracks in the Key Editor, the sync region tool allows you to push the data from a region shown in the primary track to all of the selected secondary tracks, replacing the existing data in that region in the secondary tracks with the data from the primary track.