improve docstring of SparseMatrixCSC

[abraunst]

Give the poor user a hint about constructing these matrices by cross-referencing to spzeros and sparse.

[andreasnoack]

Maybe even mention explicitly that they are typically constructed with sparse and not SparseMatrixCSC directly.

[abraunst]

Maybe even mention explicitly that they are typically constructed with sparse and not SparseMatrixCSC directly.

Something like this?

Matrix type for storing sparse matrices in the [Compressed Sparse Column](@ref man-csc) format.
The standard way of constructing SparseMatrixCSC is through the [`sparse`](@ref) function. See
also [`spzeros`](@ref), [`sprand`](@ref).