[MINOR][DOC] Fix python variance() documentation

## What changes were proposed in this pull request?

The Python documentation incorrectly says that `variance()` acts as `var_pop` whereas it acts like `var_samp` here: https://spark.apache.org/docs/latest/api/python/pyspark.sql.html#pyspark.sql.functions.variance

It was not the case in Spark 1.6 doc but it is in Spark 2.0 doc:
https://spark.apache.org/docs/1.6.0/api/java/org/apache/spark/sql/functions.html
https://spark.apache.org/docs/2.0.0/api/java/org/apache/spark/sql/functions.html

The Scala documentation is correct: https://spark.apache.org/docs/latest/api/java/org/apache/spark/sql/functions.html#variance-org.apache.spark.sql.Column-

The alias is set on this line:
https://github.com/apache/spark/blob/v2.4.3/sql/core/src/main/scala/org/apache/spark/sql/functions.scala#L786

## How was this patch tested?
Using variance() in pyspark 2.4.3 returns:
```
>>> spark.createDataFrame([(1, ), (2, ), (3, )], "a: int").select(variance("a")).show()
+-----------+
|var_samp(a)|
+-----------+
|        1.0|
+-----------+
```

Closes apache#24895 from tools4origins/patch-1.

Authored-by: tools4origins <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: tools4origins

python/pyspark/sql/functions.py

@@ -209,14 +209,14 @@ def _():
     """
 _functions_1_6_over_column = {
     # unary math functions
-    'stddev': 'Aggregate function: returns the unbiased sample standard deviation of' +
-              ' the expression in a group.',
+    'stddev': 'Aggregate function: alias for stddev_samp.',
     'stddev_samp': 'Aggregate function: returns the unbiased sample standard deviation of' +
                    ' the expression in a group.',
     'stddev_pop': 'Aggregate function: returns population standard deviation of' +
                   ' the expression in a group.',
-    'variance': 'Aggregate function: returns the population variance of the values in a group.',
-    'var_samp': 'Aggregate function: returns the unbiased variance of the values in a group.',
+    'variance': 'Aggregate function: alias for var_samp.',
+    'var_samp': 'Aggregate function: returns the unbiased sample variance of' +
+                ' the values in a group.',
     'var_pop':  'Aggregate function: returns the population variance of the values in a group.',
     'skewness': 'Aggregate function: returns the skewness of the values in a group.',
     'kurtosis': 'Aggregate function: returns the kurtosis of the values in a group.',