cleanup, unified notation NN instead of ANN

notation.md

@@ -26,7 +26,8 @@
 | CNN  | Convolutional Neural Network |
 | DL   | Deep Learning |
 | GD   | (steepest) Gradient Descent|
-| NN   | Neural Network |
+| MLP  | Multi-Layer Perceptron, a neural network with fully connected layers |
+| NN   | Neural Network (a generic one, in contrast to, e.g., a CNN or MLP) |
 | PDE  | Partial Differential Equation |
 | PBDL | Physics-Based Deep Learning |
 | SGD  | Stochastic Gradient Descent|