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: NKashyap Chamarthy <kchamart@redhat.com>