diff --git a/tokio/src/lib.rs b/tokio/src/lib.rs
index 70ff89fbf..bd85e2851 100644
--- a/tokio/src/lib.rs
+++ b/tokio/src/lib.rs
@@ -367,6 +367,12 @@
 //! rustflags = ["--cfg", "tokio_unstable"]
 //! ```
 //!
+//! <div class="warning">
+//! The <code>[build]</code> section does <strong>not</strong> go in a
+//! <code>Cargo.toml</code> file. Instead it must be placed in the Cargo config
+//! file <code>.cargo/config.toml</code>.
+//! </div>
+//!
 //! Alternatively, you can specify it with an environment variable:
 //!
 //! ```sh