Improve doc comments

sparse_strips/vello_cpu/src/pixmap.rs

diff --git a/sparse_strips/vello_cpu/src/pixmap.rs b/sparse_strips/vello_cpu/src/pixmap.rs
index 1a007e3..ca7d847 100644
--- a/sparse_strips/vello_cpu/src/pixmap.rs
+++ b/sparse_strips/vello_cpu/src/pixmap.rs
@@ -12,7 +12,7 @@
 }
 
 impl Pixmap {
-    /// Create a new pixmap.
+    /// Create a new pixmap with the given width and height in pixels.
     pub fn new(width: usize, height: usize) -> Self {
         let buf = vec![0; width * height * 4];
         Self { width, height, buf }

sparse_strips/vello_cpu/src/render.rs

diff --git a/sparse_strips/vello_cpu/src/render.rs b/sparse_strips/vello_cpu/src/render.rs
index 6b9e1f3..f732225 100644
--- a/sparse_strips/vello_cpu/src/render.rs
+++ b/sparse_strips/vello_cpu/src/render.rs
@@ -36,7 +36,7 @@
 }
 
 impl RenderContext {
-    /// Create a new render context.
+    /// Create a new render context with the given width and height in pixels.
     pub fn new(width: usize, height: usize) -> Self {
         let wide = Wide::new(width, height);