Expand man page for --group

Specifying --group without a value inhibits any change of gid. This
feature has existed since dnsmasq-2.43, released in 2008, but without
documentation, it has been lost to time, even though it's useful
in certain circumstances, mainly to do with containers.

Thanks to Leon Busch-George for his efforts to fix this.

man/dnsmasq.8

@@ -174,12 +174,18 @@ Specify an alternate path for dnsmasq to record its process-id in. Normally /var
 .B \-u, --user=<username>
 Specify the userid to which dnsmasq will change after startup. Dnsmasq must normally be started as root, but it will drop root 
 privileges after startup by changing id to another user. Normally this user is "nobody" but that 
-can be over-ridden with this switch.
+can be over-ridden with this switch. 
 .TP
 .B \-g, --group=<groupname> 
 Specify the group which dnsmasq will run
 as. The default is "dip", if available, to facilitate access to
-/etc/ppp/resolv.conf which is not normally world readable.
+/etc/ppp/resolv.conf which is not normally world readable. If "dip" is not available, the default becomes
+the main group associated with the uid specified by
+.B --user
+, or "nobody".
+Specifying
+.B --group
+without an argument inhibits any change to the gid of the process, even to the defaults.
 .TP
 .B \-v, --version
 Print the version number.