docs: kernel_abi.py: Handle with a lazy Sphinx parser

docs: kernel_abi.py: Handle with a lazy Sphinx parser

The Sphinx docutils parser is lazy: if the content is bigger than
a certain number of lines, it silenlty stops parsing it,
producing an incomplete content. This seems to be worse on newer
Sphinx versions, like 2.0.

So, change the logic to parse the contents per input file.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/4659b60795739308e34d2d00c57ee0742a9cd2ab.1604042072.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>