Drive Joints says where and why a drive stopped short (#1169)

Drive Joints says where and why a drive stopped short.

docs/book/src/guide/assemblies.md

@@ -34,7 +34,10 @@ mechanism stays in the assembly mode it started in (an open four-bar does not fl
 crossed one). If the motion would take a loop past the point where it can still close, for
 example driving a rocker beyond its swing, the motion stops at the last position where the loop
 closes, and the drive reports `stopped_by_loop`. A loop that cannot close at all is listed
-under `conflicts` and nothing in it moves.
+under `conflicts` and nothing in it moves. The Drive Joints dialog says when a drive stops short and
+why. While it previews, a row under the values gives the value the joint reached and the reason
+(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.
 
 **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

docs/contact-sets.md

@@ -16,6 +16,12 @@ to leave the bodies just short of penetration, within the tessellated contact to
 Motion links and limits participate in the trajectory checks. Moving a body between
 components preserves its placement for contact checks through its body offset.
 
+A drive can also stop where a closed loop of joints (a four-bar, a slider-crank) can no longer
+close; it then returns `stopped_by_loop` instead, with the joint whose loop opens and the
+fraction reached. When both apply, the motion stops at whichever comes first, and only that
+one is reported. The Drive Joints dialog shows the reached value and the reason while it
+previews, and the status line repeats it after OK.
+
 The trajectory uses bounded samples of cached tessellated geometry. Spacing accounts for
 effective joint travel, angular motion, distance from joint pivots and whole-body widths
 along sampled face directions. This handles tilted thin plates while keeping curved bodies