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!