Document `destructure` handling shared parameters differently to ComponentArrays.jl

[mcabbott]

This came up here:

https://discourse.julialang.org/t/on-the-future-of-flux-destructure-and-sciml-integration/104760/4

Maybe the idea that ComponentArrays.jl treats shared arrays as independent should be mentioned as an important difference, perhaps at the end of this section:

https://fluxml.ai/Optimisers.jl/dev/#Obtaining-a-flat-parameter-vector

Edit, example here:

FluxML/Flux.jl#2405 (comment)