virsh: Enhance documentation of cpu-compare command

Signed-off-by: NJiri Denemark <jdenemar@redhat.com>
Reviewed-by: NCollin Walling <walling@linux.ibm.com>
Reviewed-by: NKashyap Chamarthy <kchamart@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>

tools/virsh.pod

@@ -589,7 +589,9 @@ Compare CPU definition from XML <file> with host CPU. The XML <file> may
 contain either host or guest CPU definition. The host CPU definition is the
 <cpu> element and its contents as printed by B<capabilities> command. The
 guest CPU definition is the <cpu> element and its contents from domain XML
-definition. For more information on guest CPU definition see:
+definition. In addition to the <cpu> element itself, this command accepts
+full domain or capabilities XML containing the <cpu> element. For more
+information on guest CPU definition see:
 L<https://libvirt.org/formatdomain.html#elementsCPU>. If I<--error> is
 specified, the command will return an error when the given CPU is
 incompatible with host CPU and a message providing more details about the