docs: correct three header comments that had drifted from the code

include/plot/render/style.hpp

@@ -53,8 +53,12 @@ struct Style {
   /// (`Trace`'s default, 0x40C0FF) that a hue-based reading cannot tell from a cyan crosshair --
   /// only lightness would separate them, which no reader can rely on. A near-white can never be a
   /// saturated trace of *any* hue, and it is still plainly not `cursor`'s orange, which is what
-  /// the spec asks for. The 1 px stroke is kept: form (a full-height thin line over a polyline) is
-  /// already distinct, so no dashes.
+  /// the spec asks for.
+  ///
+  /// The stroke is thin and dashed by default -- see `crosshair_width`, `crosshair_dash_on` and
+  /// `crosshair_dash_off` below for the values: thinner than a trace so the instrument reads as
+  /// chrome rather than data, and dashed so it is distinguishable by *form* as well as by
+  /// lightness.
   Color crosshair = Color::hex(0xE6EBF2);
 
   /// The crosshair's stroke and dash, in logical pixels (spec §17). The crosshair is a