sdk5 15.2's app_timer.h at line 61 says:
* @details Use the USE_SCHEDULER parameter of the APP_TIMER_INIT() macro to select if the
* @ref app_scheduler should be used or not. Even if the scheduler is
* not used, app_timer.h will include app_scheduler.h, so when
* compiling, app_scheduler.h must be available in one of the compiler include paths.
Is the reference cruft from an earlier SDK superseded by the APP_TIMER_CONFIG_USE_SCHEDULER knob?
The same reference is in the html docs at https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v15.2.0%2Fgroup__app__scheduler.html
Thanks, Howard