[SPARK-16772][PYTHON][DOCS] Restore "datatype string" to Python API docstrings

## What changes were proposed in this pull request?

This PR corrects [an error made in an earlier PR](https://github.com/apache/spark/pull/14393/files#r72843069).

## How was this patch tested?

```sh
$ ./dev/lint-python
PEP8 checks passed.
rm -rf _build/*
pydoc checks passed.
```

I also built the docs and confirmed that they looked good in my browser.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #14408 from nchammas/SPARK-16772.