Various ML programming guide cleanups post 1.5

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.