BUG: Categorical.searchsorted(): use provided categorical order
 - [x] closes #14522
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Previously, it used lexical order instead of the provided categorical
order.

Tests updated accordingly.

Closes https://github.com/pandas-dev/pandas/issues/14522

Documentation also needs to be updated. 
Could you please review? If it's OK I'll append doc update to this pull request.
Thanks!
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14697?src=pr) is 84.57% (diff: 100%)
> Merging [#14697](https://codecov.io/gh/pandas-dev/pandas/pull/14697?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14697   diff @@
==========================================
  Files           144        144          
  Lines         51057      51059     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43180      43183     +3   
+ Misses         7877       7876     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [f1cfe5b...86b42d0](https://codecov.io/gh/pandas-dev/pandas/compare/f1cfe5b4eb614f9b4e201e764e9bd25f2389b355...86b42d08b9da389d05a83495c7d3059c9cc81b2c?src=pr) add a whatsnew note in 0.19.2 can you rebase lgtm. minor doc-changes. pls also review categorical.rst if any changes are needed. thanks! Sorry I was unavailable for the last two weeks..
Thank you!