doc: add detailed scheduler documentation

Eh, it the other way around: ARM7 checks `sched_context_switch_request`, too and then calls `sched_run()` from the interrupt handler. On all other platforms the ISR has to trigger `sched_run`, e.g. by calling `thread_yield()`. The reason for that is that the ISRs on ARM7 are mapped to a single ISR handler (written in ASM) that then calls the VIC, function. But there is no such central ISR function for the other platforms.