Documentation: Avoid creating man pages in source tree

Currently kernel-doc generates a dummy DocBook file when asked to
convert a C source file with no structured comments.  For an
out-of-tree build (objtree != srctree), the title of the output file
is the absolute path name of the C source file, which later results
in a manual page being created alongside the C source file.

Change the title to be a relative path.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>