Motion links carry a value through a chain stored in any order (#1203)

Motion links carry a value through a chain stored in any order.

docs/book/src/guide/assemblies.md

@@ -22,7 +22,9 @@ records are reported. Unsupported destructive links refuse the entire copy. See
 circles and vertices: rigid, revolute, slider, cylindrical, pin-slot, planar and ball, with
 offsets, flip and limits. **As-built Joint** keeps the current position, **Rigid Group** locks
 components together, **Drive Joints** moves a joint through its values, and **Motion Link**
-couples two joints. Contact sets stop motion where bodies would pass through each other,
+couples two joints. Links can be chained (one joint drives a second, which drives a
+third), and a value is carried through a chain of up to 64 links in whatever order they were
+made. Contact sets stop motion where bodies would pass through each other,
 **Motion Study** keys joint values along a timeline, and **Interference** shows overlapping
 volumes in red.