Add reordering example to central docs.

Author: calder

library/core/src/primitive_docs.rs

@@ -1331,6 +1331,19 @@ mod prim_f16 {}
 /// different results even within a single program run. **Unsafe code must not rely on any property
 /// of the return value for soundness.** However, implementations will generally do their best to
 /// pick a reasonable tradeoff between performance and accuracy of the result.
+///
+/// For example:
+///
+/// ```
+/// x = x.algebraic_add(x, a.algebraic_mul(b));
+/// ```
+///
+/// May be rewritten as either:
+///
+/// ```
+/// x = x + (a * b); // As written
+/// x = (a * b) + x; // Reordered to allow using a single `fma`
+/// ```
 
 #[stable(feature = "rust1", since = "1.0.0")]
 mod prim_f32 {}