schedinfo: update man page about virsh schedinfo command

This patch updates the man page about virsh schedinfo command.

 - fix typo: 1844674407370955 -> 18446744073709551
 - describe the value 0 of vcpu_period and vcpu_quota parameters
Signed-off-by: NTaku Izumi <izumi.taku@jp.fujitsu.com>

tools/virsh.pod

@@ -737,8 +737,9 @@ B<Note>: The weight and cap parameters are defined only for the
 XEN_CREDIT scheduler and are now I<DEPRECATED>.
 
 B<Note>: The vcpu_period parameter has a valid value range of 1000-1000000 or
-0, and the vcpu_quota parameter has a valid value range of 1000-1844674407370955
-or less than 0.
+0, and the vcpu_quota parameter has a valid value range of
+1000-18446744073709551 or less than 0. The value 0 for either parameter is
+the same as not specifying that parameter.
 
 =item B<screenshot> I<domain-id> [I<imagefilepath>] [I<--screen> B<screenID>]