Math: improve docs for vector cross/dot product.

Comes handy. In my case to prevent more embarassing mistakes in the
future.

src/Magnum/Math/Vector.h

@@ -92,9 +92,9 @@ template<std::size_t size, class T> class Vector {
         /**
          * @brief Dot product
          *
-         * Returns `0` if two vectors are orthogonal, `1` if two *normalized*
+         * Returns `0` when two vectors are perpendicular, `1` when two
-         * vectors are parallel and `-1` if two *normalized* vectors are
+         * *normalized* vectors are parallel and `-1` when two *normalized*
-         * antiparallel. @f[
+         * vectors are antiparallel. @f[
          *      \boldsymbol a \cdot \boldsymbol b = \sum_{i=0}^{n-1} \boldsymbol a_i \boldsymbol b_i
          * @f]
          * @see @ref dot() const, @ref operator-(), @ref Vector2::perpendicular()

src/Magnum/Math/Vector2.h

@@ -88,7 +88,9 @@ template<class T> class Vector2: public Vector<2, T> {
          * 2D version of cross product, also called perp-dot product,
          * equivalent to calling @ref Vector3::cross() with Z coordinate set to
          * `0` and extracting only Z coordinate from the result (X and Y
-         * coordinates are always zero). @f[
+         * coordinates are always zero). Returns `0` either when one of the
+         * vectors is zero or they are parallel or antiparallel and `1` when
+         * two *normalized* vectors are perpendicular, @f[
          *      \boldsymbol a \times \boldsymbol b = \boldsymbol a_\bot \cdot \boldsymbol b = a_xb_y - a_yb_x
          * @f]
          * @see @ref perpendicular(),

src/Magnum/Math/Vector3.h

@@ -104,7 +104,9 @@ template<class T> class Vector3: public Vector<3, T> {
         /**
          * @brief Cross product
          *
-         * Done using the following equation: @f[
+         * Result has length of `0` either when one of them is zero or they are
+         * parallel or antiparallel and length of `1` when two *normalized*
+         * vectors are perpendicular. Done using the following equation: @f[
          *      \boldsymbol a \times \boldsymbol b = \begin{pmatrix} c_y \\ c_z \\ c_x \end{pmatrix} ~~~~~
          *      \boldsymbol c = \boldsymbol a \begin{pmatrix} b_y \\ b_z \\ b_x \end{pmatrix} -
          *                      \boldsymbol b \begin{pmatrix} a_y \\ a_z \\ a_x \end{pmatrix}