Update docs for `vello_cpu::RenderMode` (#1322)

- Follow-up to #1294

sparse_strips/vello_cpu/src/lib.rs

diff --git a/sparse_strips/vello_cpu/src/lib.rs b/sparse_strips/vello_cpu/src/lib.rs
index 28dbf4b..497cb1f 100644
--- a/sparse_strips/vello_cpu/src/lib.rs
+++ b/sparse_strips/vello_cpu/src/lib.rs
@@ -145,6 +145,7 @@
 pub use vello_common::{color, kurbo, peniko};
 
 /// The selected rendering mode.
+/// For using [`RenderMode::OptimizeQuality`] you also need to enable `f32_pipeline` feature.
 #[derive(Copy, Clone, Debug, Default)]
 pub enum RenderMode {
     /// Optimize speed (by performing calculations with u8/16).