[SPARK-23231][ML][DOC] Add doc for string indexer ordering to user guide (also to RFormula guide)

[zhengruifeng]

What changes were proposed in this pull request?

add doc for string indexer ordering

How was this patch tested?

existing tests

[SparkQA]

Test build #93169 has finished for PR 21792 at commit febd66f.

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

[SparkQA]

Test build #93170 has finished for PR 21792 at commit 2003324.

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

[srowen]

My only question here is whether it's worth duplicating this documentation from the Python and R docs. It's more detailed info for callers of the API, and might just belong in the API doc.

[zhengruifeng]

@srowen I think we need to update the docs
1, Current doc in StringIndexer is somewhat misleading: "The indices are in [0, numLabels), ordered by label frequencies, so the most frequent label gets index 0." this is true only with default ordering type.
2, In RFormula, stringOrderType only affect feature columns, not label column. This need to be emphasised, which is somewhat out of expectation.

@MLnick your thoughts?

[srowen]

Merged to master