docs: Update docs for cpu_shares setting

* tools/virsh.pod

tools/virsh.pod

@@ -566,7 +566,9 @@ Xen (credit scheduler): weight, cap
 
 ESX (allocation scheduler): reservation, limit, shares
 
-B<Note>: The cpu_shares parameter has a valid value range of 0-262144.
+B<Note>: The cpu_shares parameter has a valid value range of 0-262144; Negative
+values are wrapped to positive, and larger values are capped at the maximum.
+Therefore, -1 is a useful shorthand for 262144.
 
 B<Note>: The weight and cap parameters are defined only for the
 XEN_CREDIT scheduler and are now I<DEPRECATED>.