doc: LearningSet - add missing class description
doc: FeedForwardLayer - add missing description of class and methods
doc: FeedForwardNetwork - add missing comments
doc: FeedForwardTeacher - add missing comments
doc: Input - add missing comment
doc: LinearTransferFunction - add missing comments
doc: Network - add missing comments
doc: Neuron - add missing comments
doc: Pixel - add missing comments
doc: PixelMatrix - add missing comments
doc: SigmoidTransferFunction - add missing comments
doc: StepTransferFunction - add missing comments
doc: TanhTransferFunction - add missing comments
doc: Teacher - add missing comments
doc: TeacherController - add missing comments
doc: TransferFunction - add missing comments