doc: Untrim's face points pick faces of the body it untrims

FeatureKind::face_body did not name Untrim's body, so its face was looked up by
name on every body and could be taken from another one (a copy has the same
face names).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/feature_geometry.rs

@@ -55,7 +55,7 @@ impl FeatureKind {
 
     /// The body a feature that works on one body's faces names (shell, draft, split, offset,
     /// delete and replace face, surface offset, the placed plastic features, sheet metal
-    /// conversion, ruled surface): its face points pick faces of that body.
+    /// conversion, ruled surface, untrim): its face points pick faces of that body.
     pub fn face_body(&self) -> Option<&str> {
         match self {
             FeatureKind::Shell { body, .. }
@@ -69,6 +69,7 @@ impl FeatureKind {
             | FeatureKind::SnapFit { body, .. }
             | FeatureKind::Rest { body, .. } => body.as_deref(),
             FeatureKind::SheetConvert { body, .. } | FeatureKind::SurfaceRuled { body, .. } => Some(body.as_ref()),
+            FeatureKind::SurfaceUntrim { face, .. } => Some(face.body.as_ref()),
             _ => None,
         }
     }