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.
lgtm
 ## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14686?src=pr) is 85.21% (diff: 100%)
> Merging [#14686](https://codecov.io/gh/pandas-dev/pandas/pull/14686?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14686   diff @@
==========================================
  Files           143        143          
  Lines         50796      50800     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43285      43288     +3   
- Misses         7511       7512     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [880de30...ad7da32](https://codecov.io/gh/pandas-dev/pandas/compare/880de3054c7ce3a6c48e295837207787dc73350f...ad7da322303251dcd06c0531d39f37cbfd1d5171?src=pr) lgtm. tough travis is not passing. @jreback : One of the build machines appears to have broke for a reason unrelated to this PR.  Are you able to restart the build on your end? @jreback : Everything is passing.  Ready to merge if there is nothing else. thanks!