Say what the code does, not what it replaced

The timings and the runs that never finished belong in the commit
messages, where they can be read against the change; a reader of the
code cannot check them. Kept the cost that still explains the design.

MultiPoint also spells out the consequence: a moved-from Polygon or
Polyline is now really empty where it used to silently keep its points.

The two wall spacing comments the parallel loop reindented are plain
ASCII now, so the whole file is.

src/libslic3r/PerimeterGenerator.cpp

@@ -2754,10 +2754,10 @@ void PerimeterGenerator::process_arachne()
                     // Get searching thresholds. For an external perimeter we take the external perimeter spacing/2 plus the internal perimeter spacing/2 and expand by the factor
                     // rounding errors. When precise wall is enabled, the external perimeter full spacing is used.
                     coord_t threshold_external = (apply_precise_outer_wall)
-                        // Precise outer wall ⇒ use “full external spacing”
+                        // Precise outer wall: use the full external spacing
                         ? ( this->ext_perimeter_flow.scaled_spacing()
                             + this->perimeter_flow.scaled_spacing()/2.0 )
-                        // Normal ⇒ half ext spacing + half int spacing
+                        // Normal: half ext spacing plus half int spacing
                         : ( this->ext_perimeter_flow.scaled_spacing()/2.0
                             + this->perimeter_flow.scaled_spacing()/2.0 );