bpf: Document optval > PAGE_SIZE behavior for sockopt hooks

Extend existing doc with more details about requiring ctx->optlen = 0
for handling optval > PAGE_SIZE.

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200617010416.93086-3-sdf@google.com