[SPARK-15256] [SQL] [PySpark] Clarify DataFrameReader.jdbc() docstring

This PR:
* Corrects the documentation for the `properties` parameter, which is supposed to be a dictionary and not a list.
* Generally clarifies the Python docstring for DataFrameReader.jdbc() by pulling from the [Scala docstrings](https://github.com/apache/spark/blob/b28137764716f56fa1a923c4278624a56364a505/sql/core/src/main/scala/org/apache/spark/sql/DataFrameReader.scala#L201-L251) and rephrasing things.
* Corrects minor Sphinx typos.

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

Closes #13034 from nchammas/SPARK-15256.