Document some static functions as non-static

[mglisse]

static Simplex_key key(Simplex_handle sh) {
The fact that the function is static is an implementation detail, because it does not use this, but we still want users to call the function with st.key(sh), not ST::key(sh), so we should document that.
Other projects use #ifdef DOXYGEN_RUNNING to enable different code with doxygen vs a real compiler.