[librsvg: 13/38] Docs on ViewBox to remind me that it is a new coordinate space

[Federico Mena Quintero <federico src gnome org>]

commit 1c7f1aa2dcd318ac070f95be705ce9d49bd9bb56
Author: Federico Mena Quintero <federico gnome org>
Date:   Tue Sep 22 09:44:06 2020 -0500

    Docs on ViewBox to remind me that it is a new coordinate space

 rsvg_internals/src/viewbox.rs | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/rsvg_internals/src/viewbox.rs b/rsvg_internals/src/viewbox.rs
index eff5a3a3..b3d40f5d 100644
--- a/rsvg_internals/src/viewbox.rs
+++ b/rsvg_internals/src/viewbox.rs
@@ -9,6 +9,9 @@ use crate::rect::Rect;
 
 /// Newtype around a [`Rect`], used to represent the `viewBox` attribute.
 ///
+/// A `ViewBox` is a new user-space coordinate system mapped onto the rectangle defined by
+/// the current viewport.  See https://www.w3.org/TR/SVG2/coords.html#ViewBoxAttribute
+///
 /// [`Rect`]: rect/type.Rect.html
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub struct ViewBox(pub Rect);