Add pairs for doxygen formulas in C / C++

[ax487]

There is a little-known feature inside doxygen, which allows users to add inline formulas to the documentation:

  // The distance between \f$(x_1,y_1)\f$ and \f$(x_2,y_2)\f$ is 
  // \f$\sqrt{(x_2-x_1)^2+(y_2-y_1)^2}\f$.

This patch adds the corresponding pairs to the sp-c-modes.

[Fuco1]

Thanks! I didn't know about this either :)