ping^5 [docs] Merge trees and generic chapters in internal docs

[Gerald Pfeifer]

On Wed, 10 Mar 2010, Jerry Quinn wrote:
> http://gcc.gnu.org/ml/gcc-patches/2010-02/msg00222.html
> 
> The patch merges the chapters on trees and generic into a more
> consistent single chapter, and separates GENERIC trees from the ones
> specific to C++ in a hopefully more logical manner.

I'm fine with this in principle wearing my doc hat, but really feel it 
would be better for a middle-end oder global maintainer to review.

Gerald