Add note about numeric domain names to manpage

Clarify that domains with numeric names can only be identified by
their domain id.

tools/virsh.pod

@@ -26,12 +26,16 @@ The basic structure of most virsh usage is:
 
   virsh [OPTION]... <command> <domain> [ARG]...
 
-Where I<command> is one of the commands listed below, I<domain> is the numeric
-domain id, or the domain name, or the domain UUID and I<ARGS> are command
-specific options.  There are a few exceptions to this rule in the cases where
-the command in question acts on all domains, the entire machine, or directly
-on the xen hypervisor.  Those exceptions will be clear for each of those
-commands.
+Where I<command> is one of the commands listed below; I<domain> is the
+numeric domain id, or the domain name, or the domain UUID; and I<ARGS>
+are command specific options.  There are a few exceptions to this rule
+in the cases where the command in question acts on all domains, the
+entire machine, or directly on the xen hypervisor.  Those exceptions
+will be clear for each of those commands.  Note: it is permissible to
+give numeric names to domains, however, doing so will result in a
+domain that can only be identified by domain id. In other words, if a
+numeric value is supplied it will be interpreted as a domain id, not
+as a name.
 
 The B<virsh> program can be used either to run one I<COMMAND> by giving the
 command and its arguments on the shell command line, or a I<COMMAND_STRING>