[DOC] lto kwarg for the @cuda.jit decorator is undocumented

[gmarkall]

#23 added an lto kwarg to the @cuda.jit decorator in an indirect fashion - the lto kwarg was added to _Kernel.__init_(), which indirectly propagates up to the jit function through **targetoptions to the CUDADispatcher and as **kws in the jit() function.

The lto kwarg needs documenting in the @jit decorator, and potentially making more explicit as well.