Clarify use cases for DuplicatedNoNeed (#1753)

lib/EnzymeCore/src/EnzymeCore.jl

@@ -79,6 +79,13 @@ end
 
 Like [`Duplicated`](@ref), except also specifies that Enzyme may avoid computing
 the original result and only compute the derivative values.
+
+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
+`DuplicatedNoNeed` may result in incorrect derivatives. In particular, `DuplicatedNoNeed`
+should not be used for preallocated workspace, even if the user might not care about its
+final value, as marking a variable as NoNeed means that reads from the variable are now
+undefined.
 """
 struct DuplicatedNoNeed{T} <: Annotation{T}
     val::T