get_abi.pl: seek for all occurrences for Documentation/ABI

Instead of retrieving just one match at most, ensure that the entire
description will be parsed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/17019b73e106d1b1b353b8880ed189bad3604c13.1616668017.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>