[SPARK-17017][ML][MLLIB][ML][DOC] Updated the ml/mllib feature selection docs for ChiSqSelector

[lins05]

What changes were proposed in this pull request?

A follow up for #14597 to update feature selection docs about ChiSqSelector.

How was this patch tested?

Generated html docs. It can be previewed at:

• ml: http://sparkdocs.lins05.pw/spark-17017/ml-features.html#chisqselector
• mllib: http://sparkdocs.lins05.pw/spark-17017/mllib-feature-extraction.html#chisqselector

[srowen]

Please add this to #15214 with @yanboliang -- this is getting spread across too many places.

[lins05]

Just found #15214 and #15212, I think this one need to wait until those are merged.

[SparkQA]

Test build #65886 has finished for PR 15236 at commit 4a2cf5e.

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

[srowen]

@lins05 OK, #15214 is merged. I don't think this can relate to #15212 because it's not necessarily going to be added. Is this doc change still correct/relevant?

[lins05]

@srowen I saw there was a proposal to change setAlpha to setFpr in #15214, but it was not changed when the PR is merged. So I think this PR is up to upate.

[srowen]

Merged to master