MAINTAINERS: clarify responsibility for checkpatch documentation

As discussed, Dwaipayan and Lukas take the responsibility for maintaining
the checkpatch documentation that is currently being built up.

To be sure that the checkpatch maintainers and the corresponding
documentation maintainers can keep the content synchronized, add them as
reviewers to the counterpart.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
Link: https://lore.kernel.org/lkml/bcee822d1934772f47702ee257bc735c8f467088.camel@perches.com/
Link: https://lore.kernel.org/r/20210226093827.12700-4-dwaipayanray1@gmail.com


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