DOC: astype now takes dict mapping col names to datatypes (#14761)
 - [x] closes https://github.com/pandas-dev/pandas/issues/14761
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

Updating documentation to reflect change
## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14837?src=pr) is 84.64% (diff: 100%)
> Merging [#14837](https://codecov.io/gh/pandas-dev/pandas/pull/14837?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

```diff
@@             master     #14837   diff @@
==========================================
  Files           144        144          
  Lines         51016      51016          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43185      43184     -1   
- Misses         7831       7832     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8e630b6...b9414a9](https://codecov.io/gh/pandas-dev/pandas/compare/8e630b63010ffdb4ed3c9b47309dc237f8141fdc...b9414a92e27ba9fe70ffba5e4167f0bdb6441791?src=pr) @m-charlton you don't need to close this PR or remove the branch on github to update this. You can just push to the same branch, and the PR on github will automatically be updated.
If you restore the branch, reopen the PR, and then push again, it should still work like that. @m-charlton Some git help, seems something went wrong with updating to master. If you do the following, it should normally be solved:

```
git fetch upstream
git checkout update_docs_astype_with_dict_#14761
git rebase upstream/master
git push -f 
``` I performed the above before I saw your git help.
Has this fixed the problem? I think I've rectified my mistakes in update b9414a9 lgtm. ping on green. I think it was more to do with the content so @jorisvandenbossche needs to
give it a look.