## What changes were proposed in this pull request?
In PySpark API Document, [SparkSession.build](http://spark.apache.org/docs/2.2.0/api/python/pyspark.sql.html) is not documented and shows default value description.
```
SparkSession.builder = <pyspark.sql.session.Builder object ...
```
This PR adds the doc.
The following is the diff of the generated result.
```
$ diff old.html new.html
95a96,101
> <dl class="attribute">
> <dt id="pyspark.sql.SparkSession.builder">
> <code class="descname">builder</code><a class="headerlink" href="#pyspark.sql.SparkSession.builder" title="Permalink to this definition">¶</a></dt>
> <dd><p>A class attribute having a <a class="reference internal" href="#pyspark.sql.SparkSession.Builder" title="pyspark.sql.SparkSession.Builder"><code class="xref py py-class docutils literal"><span class="pre">Builder</span></code></a> to construct <a class="reference internal" href="#pyspark.sql.SparkSession" title="pyspark.sql.SparkSession"><code class="xref py py-class docutils literal"><span class="pre">SparkSession</span></code></a> instances</p>
> </dd></dl>
>
212,216d217
< <dt id="pyspark.sql.SparkSession.builder">
< <code class="descname">builder</code><em class="property"> = <pyspark.sql.session.SparkSession.Builder object></em><a class="headerlink" href="#pyspark.sql.SparkSession.builder" title="Permalink to this definition">¶</a></dt>
< <dd></dd></dl>
<
< <dl class="attribute">
```
## How was this patch tested?
Manual.
```
cd python/docs
make html
open _build/html/pyspark.sql.html
```
Author: Dongjoon Hyun <dongjoon@apache.org>
Closes#19726 from dongjoon-hyun/SPARK-22490.
aa88b8dbbb
