[SPARK-10512][PySpark] Fix @since when a function doesn't have doc

[yu-iskw]

cc @mengxr @davies

[SPARK-10512] Fix @since when a function doesn't have doc - ASF JIRA

[SparkQA]

Test build #42196 has finished for PR 8667 at commit deda03a.

• 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):

[yu-iskw]

As we discussed here #8657 (comment), we should add a doc for those public API, instead putting a workaround in @SInCE.