virsh: man: Document behavior of some blkdeviotune's flags when querying

--live and --config can't be specified together when querying the
configuration, but are valid when setting. The man page was hinting that
they are valid always.

tools/virsh.pod

@@ -1100,9 +1100,10 @@ cannot be mixed with non-zero values for read or write.
 If I<--live> is specified, affect a running guest.
 If I<--config> is specified, affect the next boot of a persistent guest.
 If I<--current> is specified, affect the current guest state.
-Both I<--live> and I<--config> flags may be given, but I<--current> is
-exclusive. If no flag is specified, behavior is different depending
-on hypervisor.
+When setting the disk io parameters both I<--live> and I<--config> flags may be
+given, but I<--current> is exclusive. For querying only one of I<--live>,
+I<--config> or I<--current> can be specified. If no flag is specified, behavior
+is different depending on hypervisor.
 
 =item B<blockjob> I<domain> I<path> { [I<--abort>] [I<--async>] [I<--pivot>] |
 [I<--info>] [I<--raw>] [I<--bytes>] | [I<bandwidth>] }