rpc: fix documentation for negotiation procedure.
rpc: fix documentation for negotiation procedure.

Fix it to mention that a client does same checks as a server during
negotiation.

dreamsxin/seastar,avikivity/seastar,kjniemi/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,kjniemi/seastar,sjperkins/seastar,dreamsxin/seastar,cloudius-systems/seastar,sjperkins/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,sjperkins/seastar,scylladb/seastar,kjniemi/seastar