[SPARK-10595] [ML] [MLLIB] [DOCS] Various ML guide cleanups

Various ML guide cleanups.

* ml-guide.md: Make it easier to access the algorithm-specific guides.
* LDA user guide: EM often begins with useless topics, but running longer generally improves them dramatically.  E.g., 10 iterations on a Wikipedia dataset produces useless topics, but 50 iterations produces very meaningful topics.
* mllib-feature-extraction.html#elementwiseproduct: “w” parameter should be “scalingVec”
* Clean up Binarizer user guide a little.
* Document in Pipeline that users should not put an instance into the Pipeline in more than 1 place.
* spark.ml Word2Vec user guide: clean up grammar/writing
* Chi Sq Feature Selector docs: Improve text in doc.

CC: mengxr feynmanliang

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #8752 from jkbradley/mlguide-fixes-1.5.

1 files changed, 4 insertions, 0 deletions

diff --git a/docs/mllib-clustering.md b/docs/mllib-clustering.md
index 3fb35d3c50..c2711cf82d 100644
--- a/docs/mllib-clustering.md
+++ b/docs/mllib-clustering.md
@@ -507,6 +507,10 @@ must also be $> 1.0$. Providing `Vector(-1)` results in default behavior
 $> 1.0$. Providing `-1` results in defaulting to a value of $0.1 + 1$.
 * `maxIterations`: The maximum number of EM iterations.
 
+*Note*: It is important to do enough iterations.  In early iterations, EM often has useless topics,
+but those topics improve dramatically after more iterations.  Using at least 20 and possibly
+50-100 iterations is often reasonable, depending on your dataset.
+
 `EMLDAOptimizer` produces a `DistributedLDAModel`, which stores not only
 the inferred topics but also the full training corpus and topic
 distributions for each document in the training corpus. A