Second fix in docs needed for https://github.com/Itseez/opencv_contrib/pull/70
http://www.build.opencv.org/buildbot/builders/precommit-contrib_docs/builds/277

Apart from not displaying the table, it breaks the doc build of https://github.com/Itseez/opencv_contrib/pull/70.

- Reworked documentation to reflect actual code
- Removed some unused variables
- Removed unnecessary 'cv::' modifiers

- Extracted common operations to separate functions.
- Activated first parameters set.
- Some output formatting.
- Fixed loop break condition in mini_batch_gradient function.

fixed warnings in type conversions from size_t to int (in getting size of number of unique classes in a training problem).