document iputils socket access changes
As seen in https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1103502 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1089626 (among others), the changes to access control for the network sockets used by ping
can be disruptive. This MR documents the changes and explains possible solutions for impacted users.