getsockopt.2: Clarify the SO_REUSEPORT_LB text a bit

Refer to sockets rather than processes, since one can have multiple
sockets in a load-balancing group within the same process.

MFC after: 1 week
Sponsored by: Modirum MDPay
Sponsored by: Klara, Inc.

(cherry picked from commit d0f8e31761b7de6010e8b3ed49a1a5f9fc5726f7)