Add structured matrix notes to `ldiv!`/`rdiv!` docstring

[jishnub]

Certain structured matrix types are actually permitted as arguments to ldiv! and rdiv! despite the admonition against using matrices as arguments, so it's best to be clear about this. E.g.

julia> U = UpperTriangular(rand(2,2))
2×2 UpperTriangular{Float64, Matrix{Float64}}:
 0.303616  0.129732
  ⋅        0.269387

julia> ldiv!(U, rand(2))
2-element Vector{Float64}:
 1.6539496185029805
 0.061592851259199104