DEPR: Patch to_dense behaviour for sparse.
Patches the following for `to_dense`:

1) Fix `SparseArray.to_dense` documentation to refer to `SparseArray` and not `SparseSeries`.

2) Deprecate the fill parameter in `SparseArray.to_dense`, as that parameter was not being respected.

3) Deprecate the sparse_only parameter in `SparseSeries.to_dense`, as that parameter is inconsistent with the `to_dense` API we want, which is no parameters.

Closes #14647.