19610: drivers/periph/rtc: improve doc on rtc_set_alarm r=maribu a=maribu
### Contribution description
- point out behavior on denormalized time stamps
- use errno codes to indicate errors (and adapt the few instances of actual error handling to use them)
19670: cpu/stm32: stm32f4 BRR from BSRR r=maribu a=kfessel
### Contribution description
sometimes one wants to save one instruction :)
just write the bits we need to write.
### Testing procedure
tests/periph/gpio_ll tests this
### Issues/PRs references
`@maribu` might know some reference
maybe #19407
19678: gnrc_sixlowpan_iphc: fix NULL pointer dereference r=maribu a=miri64
19679: gnrc_sixlowpan_frag_sfr: fix ARQ scheduler race-condition r=maribu a=miri64
19680: gnrc_sixlowpan_frag_rb: fix OOB write in _rbuf_add r=maribu a=miri64
19681: sys/xtimer: improve documentation r=maribu a=maribu
### Contribution description
- Add a warning that xtimer is deprecated, so that new code hopefully starts using ztimer
- Add a hint that `ztimer_xtimer_compat` can be used even after `xtimer` is gone
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Co-authored-by: Karl Fessel <karl.fessel@ovgu.de>
Co-authored-by: Martine Lenders <m.lenders@fu-berlin.de>
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