Improve explanation regarding the notation (Section 3.7)

Since ArchiMate 3.2 all elements have two notations (boxed and icon). The rephrased explanation below makes it clearer:

"This standard notation for elements consists of a box with an icon in the upper-right corner (the boxed notation) or the icon in itself (the icon notation). Both notations can be used indistinctly and mixed in a view. It’s recommended to use this standard iconography whenever possible so that anyone familiar with the ArchiMate language can read the diagrams produced in it."