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

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:

https://github.com/FluxML/Flux.jl/issues/2405#issuecomment-2015447460

Start with the “Obtaining a flat parameter vector” section in the Optimisers.jl documentation, then read the linked Discourse discussion and Flux.jl example. Document how destructure handles shared arrays differently from ComponentArrays.jl, with the difference and its implications clearly stated in that section.

Written by the indexing model from the issue text.