Added explanation of why doctests were needed

Author: CalebVL

src/sage/matrix/matrix_dense.pyx

@@ -279,6 +279,8 @@ cdef class Matrix_dense(matrix.Matrix):
 
         TESTS:
 
+        Verify that :issue:`15067` is fixed::
+        
             sage: u = matrix(1, 2, [-1, 1])
             sage: derivative(u, x)
             [0 0]

src/sage/matrix/matrix_sparse.pyx

@@ -814,6 +814,8 @@ cdef class Matrix_sparse(matrix.Matrix):
 
         TESTS:
 
+        Verify that :issue:`15067` is fixed::
+
             sage: m = matrix(3, 3, {(1, 1): 2, (0,2): 5})
             sage: derivative(m, x)
             [0 0 0]