Document why re-aliasing is important

[lassepe]

The documentation points out, that it really important to re-alias the result, e.g. in

result = ForwardDiff.hessian!(result, f, x);

It says that more information for why this is done can be found in the documentation of ForwardDiff.hessian!. Unfortunately, the according documentation does not talk about DiffResults at all so I remain curious why re-aliasing is necessary here. Is re-aliasing no longer needed?

[gdalle]

Aliasing is still important, due to the updating code looking like this everywhere:

DiffResults.jl/src/DiffResults.jl

Lines 147 to 158 in 724a23d

	"""
	value!(r::DiffResult, x)
	Return `s::DiffResult` with the same data as `r`, except for `value(s) == x`.
	This function may or may not mutate `r`. If `r::ImmutableDiffResult`, a totally new
	instance will be created and returned, whereas if `r::MutableDiffResult`, then `r` will be
	mutated in-place and returned. Thus, this function should be called as `r = value!(r, x)`.
	"""
	value!(r::MutableDiffResult, x::Number) = (r.value = x; return r)
	value!(r::MutableDiffResult, x::AbstractArray) = (copyto!(value(r), x); return r)
	value!(r::ImmutableDiffResult{O,V}, x::Union{Number,AbstractArray}) where {O,V} = ImmutableDiffResult(convert(V, x), r.derivs)

Basically, whenever you call res2 = do_stuff!(res1, args...), you're unsure that res1 is correct: all you know is that res2 is.