Improve spi write16/read16 documentation.

[4ilo]

The write16/read16 functions only work as described after changing the amount of data_bits using spi_set_format.

[Wren6991]

Thanks! Nice catch

[scminvent]

Filed upstream after all, under the public skycmoon identity:

• Issue: pico_cyw43_arch_lwip_sys_freertos: LWIP_PROVIDE_ERRNO=1 conflicts with newlib's thread-local errno #2973
• PR (against develop): pico_cyw43_arch FreeRTOS+lwIP: route errno through newlib (LWIP_ERRNO_STDINCLUDE) #2974

The fix shape is the LWIP_PROVIDE_ERRNO=1 → LWIP_ERRNO_STDINCLUDE=1 swap on the FreeRTOS+lwIP variant only, equivalent to our existing lwipopts.h workaround. This issue stays open to track the upstream outcome; close once #2974 lands or is rejected.