Joint drives say when they move nothing; drive values in degrees; quiet replies (#1304)

joint.drive answers moved, and a reason (with unconnected) when the joint can move nothing; motion.play answers moved and unmoved; joint.list answers floating chains. joint.drive values come back in joint.list's shape (deg/mm). quiet: true trims joint answers and motion.play poses. A single value keeps a joint's other values. Drive Joints shows the floating case (drive.unconnected, all locales).

docs/book/src/guide/assemblies.md

@@ -41,6 +41,21 @@ why. While it previews, a row under the values gives the value the joint reached
 (the loop that can't close further, or the two parts a contact set keeps apart), and after OK the
 status line repeats it. The value box keeps what you typed.
 
+The solver places what it reaches from the root and from grounded components. A chain of joints
+that is joined to nothing grounded is never placed, so driving one of its joints moves nothing.
+The drive says so: `joint.drive` answers `moved: false` with a `reason` and the `unconnected`
+part ("Pedestal:1 is not connected to anything grounded"), the Drive Joints dialog shows the same
+row, `motion.play` lists such joints under `unmoved`, and `joint.list` names every such chain under
+`floating`. Ground a part of the chain, or join it to a grounded part (a rigid as-built joint keeps
+it where it is), and it moves.
+
+`joint.drive` answers the joint's values as `joint.list` gives them, in degrees and millimetres
+with their units and limits, and `moved` says whether any part moved. A joint's origins are mated
+by default: B's z axis is turned against A's, as two faces meet. `flip: true` lines them up
+instead, so two origins given at one point with the same z stay as they are. For scripts that
+drive many steps, `quiet: true` leaves the per-part degrees of freedom out of joint answers, and
+`motion.play` with `quiet: true` gives the poses of only the parts that moved.
+
 **Insert Design** (`component.insert_design {path, translate?}`) adds a component linked to
 another saved design, so each part can live in its own document. The part's solid bodies come
 in as they sit in that design, and they are kept in the assembly, so it opens even without the