docbook: skip files with no docs since they generate scary warnings

Fix docbook templates that reference files that do not contain the
expected kernel-doc notation.

Fixes these warnings:

  Warning(arch/x86/include/asm/unaligned.h): no structured comments found
  Warning(lib/vsprintf.c): no structured comments found

These cause errors in the generated html output, like below, so drop
these lines.

  Name
  arch/x86/include/asm/unaligned.h - Document generation inconsistency
  Oops
  Warning
  The template for this document tried to insert the structured comment from the file arch/x86/include/asm/unaligned.h at this point, but none was found. This dummy section is inserted to allow generation to continue.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>