add comments to the example

Author: wassup05

example/intrinsics/example_matmul.f90

@@ -1,9 +1,10 @@
 program example_matmul
     use stdlib_intrinsics, only: stdlib_matmul
     complex :: x(2, 2), y(2, 2), z(2, 2)
-    x = reshape([(0, 0), (1, 0), (1, 0), (0, 0)], [2, 2])
+
+    x = reshape([(0, 0), (1, 0), (1, 0), (0, 0)], [2, 2])  ! pauli x-matrix
     y = reshape([(0, 0), (0, 1), (0, -1), (0, 0)], [2, 2]) ! pauli y-matrix
-    z = reshape([(1, 0), (0, 0), (0, 0), (-1, 0)], [2, 2])
+    z = reshape([(1, 0), (0, 0), (0, 0), (-1, 0)], [2, 2]) ! pauli z-matrix
 
     print *, stdlib_matmul(x, y) ! should be iota*z
     print *, stdlib_matmul(y, z, x) ! should be iota*identity