Merge overloads when \overload is the only comment

[cjdb]

The current /// \overload adds some text to say that a function is an overload of something else, but it's possible to do this in raw reStructured text:

.. cpp:function::
  void f(int x);
  void f(double x);

This groups the two functions together, and puts prose text underneath both of them. I think documentation would be more readable if

/// some docs
void f(int x);

/// \overload
void f(double x);

were turned into the above rST instead of the following:

.. cpp:function:: void f(int x)

  some docs

.. cpp:function:: void f(double x)

  This is an overload...

[LecrisUT]

I would go slightly different. Merge them if there is no body. That would allow us to still specify \param and other such directives