[SPARK-16429][SQL] Include `StringType` columns in `describe()`

## What changes were proposed in this pull request?

Currently, Spark `describe` supports `StringType`. However, `describe()` returns a dataset for only all numeric columns. This PR aims to include `StringType` columns in `describe()`, `describe` without argument.

**Background**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe("age", "name").show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

**Before**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+
|summary|               age|
+-------+------------------+
|  count|                 2|
|   mean|              24.5|
| stddev|7.7781745930520225|
|    min|                19|
|    max|                30|
+-------+------------------+
```

**After**
```scala
scala> spark.read.json("examples/src/main/resources/people.json").describe().show()
+-------+------------------+-------+
|summary|               age|   name|
+-------+------------------+-------+
|  count|                 2|      3|
|   mean|              24.5|   null|
| stddev|7.7781745930520225|   null|
|    min|                19|   Andy|
|    max|                30|Michael|
+-------+------------------+-------+
```

## How was this patch tested?

Pass the Jenkins with a update testcase.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14095 from dongjoon-hyun/SPARK-16429.

1 files changed, 4 insertions, 4 deletions

diff --git a/python/pyspark/sql/dataframe.py b/python/pyspark/sql/dataframe.py
index dd670a9b3d..ab41e88620 100644
--- a/python/pyspark/sql/dataframe.py
+++ b/python/pyspark/sql/dataframe.py
@@ -751,15 +751,15 @@ class DataFrame(object):
 
     @since("1.3.1")
     def describe(self, *cols):
-        """Computes statistics for numeric columns.
+        """Computes statistics for numeric and string columns.
 
         This include count, mean, stddev, min, and max. If no columns are
-        given, this function computes statistics for all numerical columns.
+        given, this function computes statistics for all numerical or string columns.
 
         .. note:: This function is meant for exploratory data analysis, as we make no \
         guarantee about the backward compatibility of the schema of the resulting DataFrame.
 
-        >>> df.describe().show()
+        >>> df.describe(['age']).show()
         +-------+------------------+
         |summary|               age|
         +-------+------------------+
@@ -769,7 +769,7 @@ class DataFrame(object):
         |    min|                 2|
         |    max|                 5|
         +-------+------------------+
-        >>> df.describe(['age', 'name']).show()
+        >>> df.describe().show()
         +-------+------------------+-----+
         |summary|               age| name|
         +-------+------------------+-----+