Clarify the SIGPIPE NEWS entry for 3.9

Change-Id: Ice79831b32974f1abd60a9a104e136ea8de1be2b
Reviewed-on: https://gerrit.stanford.edu/1511
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>