DOC: low_memory in read_csv
- [x] closes #5888,  xref #12686
- [x] passes `git diff upstream/master | flake8 --diff`

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13293?src=pr) is **84.20%**

> Merging [#13293](https://codecov.io/gh/pydata/pandas/pull/13293?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13293   diff @@
==========================================
  Files           138        138          
  Lines         50587      50587          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          42593      42593          
  Misses         7994       7994          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [b4e2d34...282f1f3](https://codecov.io/gh/pydata/pandas/compare/b4e2d34edcbc404f6c90f76b67bcc5fe26f0945f...282f1f38cdfdcf9fa4d47e46ba2354d6e800b0a4)
 @jreback - updated
 thanks!
