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

include/plot/plot/plot.hpp

@@ -554,9 +554,11 @@ class Plot {
 
   /// A floor on the left gutter `layout()` reserves, in device pixels: the label
   /// strip and the title column together are never narrower than this, so `axes`
-  /// never starts left of `viewport().x + min_y_gutter()`. 0.0 -- the default --
-  /// imposes nothing and the plot sizes its strip from its own widest tick label,
-  /// which is what a plot standing on its own has always done.
+  /// starts no further left than `viewport().x + min_y_gutter()` **up to the cap
+  /// below** -- a floor wider than half the viewport is clamped exactly as the
+  /// measured strip is, so the guarantee is `min(min_y_gutter(), y_budget)`. 0.0
+  /// -- the default -- imposes nothing and the plot sizes its strip from its own
+  /// widest tick label, which is what a plot standing on its own has always done.
   ///
   /// It exists for `Figure`, which is the only thing that can know the answer: a row
   /// cannot see its siblings' tick labels, so the figure lays each row out once to