docs: kdoc_files: allows the caller to use a different xforms class

docs: kdoc_files: allows the caller to use a different xforms class

While the main goal for kernel-doc is to be used inside the Linux
Kernel, other open source projects could benefit for it. That's
currently the case of QEMU, which has a fork, mainly due to two
reasons:

  - they need an extra C function transform rule;
  - they handle the html output a little bit different.

Add an extra optional argument to make easier for the code to be
shared, as, with that, QEMU can just create a new derivated class
that will contain its specific rulesets, and just copy the
remaining kernel-doc files as-is.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <6b274ddbdcd9d438c6848e00e410a2f65ef80ec2.1772810574.git.mchehab+huawei@kernel.org>