DOC/TST: dtype param in read_fwf
 - [x] closes #7141
 - [ ] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

#14295 actually implemented the behavior, this is just adding docs and tests.
cc @gfyoung  LGTM! @chris-b1 Thanks!