More info for dupnoneed

lib/EnzymeCore/src/EnzymeCore.jl

@@ -79,7 +79,27 @@ end
     DuplicatedNoNeed(x, ∂f_∂x)
 
 Like [`Duplicated`](@ref), except also specifies that Enzyme may avoid computing
-the original result and only compute the derivative values.
+the original result and only compute the derivative values. This creates opportunities
+for improved performance.
+
+```jldoctest dupnoneed
+
+function square_byref(out, v)
+    out[] = v * v
+    nothing
+end
+
+out = Ref(0.0)
+dout = Ref(1.0)
+Enzyme.autodiff(Reverse, square_byref, DuplicatedNoNeed(out, dout), Active(1.0))
+dout[]
+
+# output
+0.0
+```
+
+For example, marking the out variable as `DuplicatedNoNeed` instead of `Duplicated` allows
+Enzyme to avoid computing `v * v`.
 
 This should only be used if `x` is a write-only variable. Otherwise, if the differentiated
 function stores values in `x` and reads them back in subsequent computations, using