Add Help to manual for FGB Evaluate > Classification Report

[dlogan]

Related to #231 . Add clarification to CPA Help on how to achieve the accuracy plot shown in the manual from #231. (Bottom of http://cellprofiler.org/cpa/5_classifier.html#v-b-5-find-rules-training-classifier)

Basically add a line that says what @daviddao said in #231, explaining the two options under Evaluate, and how choosing FGB affects the output: "... the Rule# vs. Accuracy plot for 'Classification Report' of FastGentleBoosting and F-Scores for sklearn classifiers..."