Hi Paul,
http://gcc.gnu.org/ml/gcc-patches/2010-09/msg00312/front-end-doc.patch
In addition to Phil's patch targeting the official GCC documentation, I also create a kind of whitepaper that tried to explain a skeleton of a GCC front-end. It is available online at: http://blog.lxgcc.net/wp-content/uploads/2011/03/GCC_frontend.pdf
I guess it is meanwhile pretty out-dated, but maybe it helps you to get along with some GCC internals.
BTW, very good idea to create a front-end documentation ;-) Best regards, Andi
