It would nice to be able to use the @relates markup (or another) to override the documentation of a friendship.  Even though a method is a friend of another I have a scenario where I would prefer to document it on another class.

For example the following code:

/**
 * First test class that should be documented.
 */
class TypeOne
{
public:

private:
  friend TypeOne toTypeOne(const TypeTwo& two);

  // Hide this constructor for internal use.
  TypeOne(int i);

};

/**
 * Second test class that should be documented.
 */
class TypeTwo
{
public:

  TypeTwo();

};

/**
 * @relates TypeTwo
 * 
 * Populates a TypeOne from a TypeTwo.
 */
TypeOne
toTypeOne(const TypeTwo& two);

will document the method toTypeOne on the class TypeOne even though the relates tag requests it be documented with TypeTwo (which is the indicated preference).