[SPARK-14498][ML][PYTHON][SQL] Many cleanups to ML and ML-related docs

[jkbradley]

What changes were proposed in this pull request?

Cleanups to documentation. No changes to code.

• GBT docs: Move Scala doc for private object GradientBoostedTrees to public docs for GBTClassifier,Regressor
• GLM regParam: needs doc saying it is for L2 only
• TrainValidationSplitModel: add .. versionadded:: 2.0.0
• Rename “_transformer_params_from_java” to “_transfer_params_from_java”
• LogReg Summary classes: “probability” col should not say “calibrated”
• LR summaries: coefficientStandardErrors —> document that intercept stderr comes last. Same for t,p-values
• approxCountDistinct: Document meaning of “rsd" argument.
• LDA: note which params are for online LDA only

How was this patch tested?

Doc build

[jkbradley]

CC @dbtsai Could you please confirm the doc changes about coefficientStandardErrors and GLM's regParam? Thanks!

[SparkQA]

Test build #55394 has finished for PR 12266 at commit e746a17.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #55395 has finished for PR 12266 at commit 56acf90.

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

[dbtsai]

Both looks good to me. Thanks. I'll go ahead and merge it soon.

[dbtsai]

Thanks. Merged into master.

[jkbradley]

Thanks for taking a look!