bpf, doc: add note on loading from .text for iproute2
bpf, doc: add note on loading from .text for iproute2

Add a note that section annotation is not always required for the
main prog.

Signed-off-by: Daniel Borkmann <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@iogearbox.net>

tgraf/cilium,michi-covalent/cilium,scanf/cilium,cilium-team/cilium,cilium/cilium,cilium/cilium,scanf/cilium,tklauser/cilium,scanf/cilium,cilium/cilium,scanf/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,michi-covalent/cilium,tgraf/cilium,scanf/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,scanf/cilium,cilium-team/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium