Document performance characteristics of Buffer::new

src/buffer/mod.rs

@@ -20,6 +20,8 @@ pub struct Buffer {
 }
 
 impl Buffer {
+    /// This is a cheap operation; you don't need to worry about reusing buffers
+    /// for efficiency.
     #[inline]
     #[cfg_attr(feature = "no-panic", no_panic)]
     pub fn new() -> Self {