Few files under dma-buf/ changed their names but the changes didn't
applied to a document that referencing them.  It is causing few
documentation build warnings.  This commit fixes the problems by
applying changed file names on the document.
Signed-off-by: NSeongJae Park <sj38.park@gmail.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

We don't need to keep it as a single large file anymore; split it up so
that it is easier to manage and the individual sections can be read
directly as plain files.
Signed-off-by: NJonathan Corbet <corbet@lwn.net>