[librsvg] Document ViewBox

[Federico Mena Quintero <federico src gnome org>]

commit f431ef06789b4c9090c1cadb50cf53ef11e2ba02
Author: Federico Mena Quintero <federico gnome org>
Date:   Mon May 4 18:26:02 2020 -0500

    Document ViewBox

 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 e075d315..eff5a3a3 100644
--- a/rsvg_internals/src/viewbox.rs
+++ b/rsvg_internals/src/viewbox.rs
@@ -7,6 +7,9 @@ use crate::number_list::{NumberList, NumberListLength};
 use crate::parsers::Parse;
 use crate::rect::Rect;
 
+/// Newtype around a [`Rect`], used to represent the `viewBox` attribute.
+///
+/// [`Rect`]: rect/type.Rect.html
 #[derive(Debug, Copy, Clone, PartialEq)]
 pub struct ViewBox(pub Rect);