[SPARK-16772] Correct API doc references to PySpark classes + formatting fixes

## What's Been Changed

The PR corrects several broken or missing class references in the Python API docs. It also correct formatting problems.

For example, you can see [here](http://spark.apache.org/docs/2.0.0/api/python/pyspark.sql.html#pyspark.sql.SQLContext.registerFunction) how Sphinx is not picking up the reference to `DataType`. That's because the reference is relative to the current module, whereas `DataType` is in a different module.

You can also see [here](http://spark.apache.org/docs/2.0.0/api/python/pyspark.sql.html#pyspark.sql.SQLContext.createDataFrame) how the formatting for byte, tinyint, and so on is italic instead of monospace. That's because in ReST single backticks just make things italic, unlike in Markdown.

## Testing

I tested this PR by [building the Python docs](https://github.com/apache/spark/tree/master/docs#generating-the-documentation-html) and reviewing the results locally in my browser. I confirmed that the broken or missing class references were resolved, and that the formatting was corrected.

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

Closes #14393 from nchammas/python-docstring-fixes.

1 files changed, 1 insertions, 1 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index 0cbb3adfa8..a986092f5d 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -196,7 +196,7 @@ class DataFrame(object):
     @property
     @since(1.3)
     def schema(self):
-        """Returns the schema of this :class:`DataFrame` as a :class:`types.StructType`.
+        """Returns the schema of this :class:`DataFrame` as a :class:`pyspark.sql.types.StructType`.
 
         >>> df.schema
         StructType(List(StructField(age,IntegerType,true),StructField(name,StringType,true)))