doc: rpc: document undocumented params
doc: rpc: document undocumented params

Resolve following warnings:
include/seastar/rpc/rpc.hh:438: warning: The following parameters of seastar::rpc::client::client(const logger &l, void *s, const socket_address &addr, const socket_address &local={}) are not documented:
include/seastar/rpc/rpc.hh:449: warning: The following parameters of seastar::rpc::client::client(const logger &l, void *s, socket socket, const socket_address &addr, const socket_address &local={}) are not documented:

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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