BUG: ufunc is not applied to sparse.fill_value
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

When ufunc is applied to sparse, it is not applied to `fill_value`. Thus results are incorrect.

**on current master:**

```
np.abs(pd.SparseArray([1, -2, -1], fill_value=-2))
# [1.0, -2, 1.0]
# Fill: -2
# IntIndex
# Indices: array([0, 2], dtype=int32)

np.add(pd.SparseArray([1, -2, -1], fill_value=-2), 1)
# [2.0, -2, 0.0]
# Fill: -2
# IntIndex
# Indices: array([0, 2], dtype=int32)
```

cc @gfyoung 

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13853?src=pr) is 85.27% (diff: 93.33%)

> Merging [#13853](https://codecov.io/gh/pydata/pandas/pull/13853?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13853   diff @@
==========================================
  Files           139        139          
  Lines         50020      50031    +11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42657      42666     +9   
- Misses         7363       7365     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [97de42a...a14f573](https://codecov.io/gh/pydata/pandas/compare/97de42abbea588fc5a46be5c58788958fd817b7f...a14f57304fdb913fb1622b7469c4734610caeb6e?src=pr)
 LGTM

cc @jreback
 I understand why this is needed, but it feels a tad unnatural. I am not sure a user will be expecting that the fill value will have the ufunc be applied here. Can we add a section to the docs showing this?
 Sure, added small section. 
