[SPARK-10249][ML][Doc]Add Python Code Example to StopWordsRemover User Guide

[hhbyyh]

jira: https://issues.apache.org/jira/browse/SPARK-10249

update user guide since python support added.

[SparkQA]

Test build #42069 has finished for PR 8620 at commit 6f7cd5f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

• show() doesn't return anything
• minor: it would be useful to call show() with truncate=False for user guide.

[mengxr]

LGTM except one minor inline comment

[hhbyyh]

Thanks for the review @mengxr

[mengxr]

Merged into master. Thanks!

[SparkQA]

Test build #42190 has finished for PR 8620 at commit 28f179e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • case class Instance(w: Double, a: Vector, b: Double)
  • class WeibullGenerator(
  • class IndexToString(JavaTransformer, HasInputCol, HasOutputCol):