Document what `issparse(A)` guarantees or requires

[jishnub]

The doumentation states that it's possible to query if an array is sparse using issparse, but it's unclear what more this implies. Does issparse(A) imply that the array extends SparseArrays functions such as nnz? Or does it imply that it's reasonable to use sparse(A) instead of A, as in the concatenation methods defined in this package? The latter would presumably also require the array to define sparse(A) to avoid slow fallbacks.