DOC: asfreq clarify original NaNs are not filled (GH9963)
Closes #9963

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13617?src=pr) is **84.33%**

> Merging [#13617](https://codecov.io/gh/pydata/pandas/pull/13617?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13617   diff @@
==========================================
  Files           138        138          
  Lines         51100      51100          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43097      43097          
  Misses         8003       8003          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [5605f99...0f8931f](https://codecov.io/gh/pydata/pandas/compare/5605f99105b6a97fb950fbebbf83d2cac0b23ef1...0f8931f20d6a507a9e0b2bfaef0b3ddce953b3cc?src=pr)
 lgtm
