docs: formatdomain: Document the CPU feature 'name' attribute
    
    Currently, the CPU feature 'name' XML attribute, as in:
    
        [...]
        <cpu match='exact'>
          <model fallback='forbid'>IvyBridge</model>
          <vendor>Intel</vendor>
          <feature policy='require' name='pcid'/>
        </cpu>
        [...]
    
    isn't explicitly documented in formatdomain.html.
    
    Document it now.
    
    Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>