virsh: Clarify use of the --managed-save flag for the list command

The documentation for the flag doesn't clearly state that the flag only
enhances the output and the user needs to specify other flags to list
inactive domains, that are enhanced by this flag.

tools/virsh.c

@@ -924,7 +924,7 @@ static const vshCmdOptDef opts_list[] = {
     {"name", VSH_OT_BOOL, 0, N_("list domain names only")},
     {"table", VSH_OT_BOOL, 0, N_("list table (default)")},
     {"managed-save", VSH_OT_BOOL, 0,
-     N_("mark domains with managed save state")},
+     N_("mark inactive domains with managed save state")},
     {"title", VSH_OT_BOOL, 0, N_("show short domain description")},
     {NULL, 0, 0, NULL}
 };

tools/virsh.pod

@@ -381,8 +381,9 @@ into s3 state.
 =back
 
 If I<--managed-save> is specified, then domains that have managed save
-state (only possible if they are in the B<shut off> state) will
-instead show as B<saved> in the listing. This flag is usable only with the
+state (only possible if they are in the B<shut off> state, so you need to
+specify I<--inactive> or I<--all> to actually list them) will instead
+show as B<saved> in the listing. This flag is usable only with the
 default I<--table> output.
 
 If I<--name> is specified, domain names are printed instead of the table