docs/bpf: update out-of-date doc in BPF flow dissector

Commit a5e2151f ("net/ipv6: SKB symmetric hash should incorporate
transport ports") removed the use of FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL
in __skb_get_hash_symmetric(), making the doc out-of-date.

Signed-off-by: Quan Tian <qtian@vmware.com>
Link: https://lore.kernel.org/r/20230911152353.8280-1-qtian@vmware.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>