bpf: Update bpf_{g,s}etsockopt() documentation

* append missing optnames to the end
* simplify bpf_getsockopt()'s doc

Signed-off-by: Ji Rongfeng <SikoJobs@outlook.com>
Link: https://lore.kernel.org/r/DU0P192MB15479B86200B1216EC90E162D6099@DU0P192MB1547.EURP192.PROD.OUTLOOK.COM
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>