doc-rst: make cleandocs misses a fair number of files

Removes intermediate 'Documentation/DocBook/.*.xml.cmd' files

Changes since v1:
- Reduce the patch to DocBook cleandocs

References: http://lkml.kernel.org/r/CA+r1Zhjr5SCVAroREBv84t9bxDVu5jVJ_Fu=BbVDGNNABdQOuQ@mail.gmail.com
Reported-by: Jim Davis <jim.epost@gmail.com>
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>