[SPARK-33252][PYTHON][DOCS] Migration to NumPy documentation style in…

… MLlib (pyspark.mllib.*)

### What changes were proposed in this pull request?

This PR proposes migration of `pyspark.mllib` to NumPy documentation style.

### Why are the changes needed?

To improve documentation style.

Before:

![old](https://user-images.githubusercontent.com/1554276/100097941-90234980-2e5d-11eb-8b4d-c25d98d85191.png)

After:

![new](https://user-images.githubusercontent.com/1554276/100097966-987b8480-2e5d-11eb-9e02-07b18c327624.png)

### Does this PR introduce _any_ user-facing change?

Yes, this changes both rendered HTML docs and console representation (SPARK-33243).

### How was this patch tested?

`dev/lint-python` and manual inspection.

Closes #30413 from zero323/SPARK-33252.

Authored-by: zero323 <mszymkiewicz@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

python/docs/source/reference/pyspark.mllib.rst

@@ -216,6 +216,8 @@ Statistics
     ChiSqTestResult
     MultivariateGaussian
     KernelDensity
+    ChiSqTestResult
+    KolmogorovSmirnovTestResult
 
 
 Tree
@@ -250,4 +252,3 @@ Utilities
     Loader
     MLUtils
     Saveable
-