Add a note about [0, 0, 0] and cogl_vector3_normalize()

When having the [0, 0, 0] vector, normalize() won't try to divide by 0
and thus it's always safe to use that funtion.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit 64370e7ddd32f25556a1792c00d14adc48a81d45)

cogl/cogl-vector.h

@@ -233,6 +233,10 @@ cogl_vector3_divide_scalar (float *vector,
  * Updates the vector so it is a "unit vector" such that the
  * @vector<!-- -->s magnitude or length is equal to 1.
  *
+ * <note>It's safe to use this function with the [0, 0, 0] vector, it will not
+ * try to divide components by 0 (its norm) and will leave the vector
+ * untouched.</note>
+ *
  * Since: 1.4
  * Stability: Unstable
  */