add warning about par_render

Author: coder543

src/bytecode.rs

@@ -75,6 +75,13 @@ impl<
 
     /// Renders a template across multiple items in parallel using Rayon with
     /// convenient internally-managed buffers, which requires a mutable reference to self.
+    ///
+    /// NOTE: This function makes serious trade-offs to enable the _maximum_ throughput.
+    /// It is far less efficient, and builds up a single buffer containing all results
+    /// prior to writing this buffer into the output, so it can consume much more memory,
+    /// and the latency to first-write is also significantly higher.
+    ///
+    /// Only use if total throughput is the sole concern.
     #[cfg(feature = "rayon")]
     pub fn par_render<'b, RunnerItem>(
         &mut self,