doc: modules guide updated with Zephyr CMake lists description

This commit extends the `Build system integration` section in the Zephyr
modules guide with a description on how a module may append additional
values to Zephyr CMake lists.

This is useful for modules that might need to append values to the
`SYSCALL_INCLUDE_DIRS` list.

This commit is followup to the discussion found in:
https://github.com/zephyrproject-rtos/zephyr/pull/33756



Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>