Documentation: add documentation of ping_group_range

Support for non-root users to send ICMP ECHO requests was added
back in Linux 3.0 kernel, but the documentation for the sysctl
to enable it has been missing.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>