`PICO_FLASH_SPI_CLKDIV` not mentioned in configuration defines

The "Full List of SDK Configuration Defines" in the C/C++ SDK document does not list `PICO_FLASH_SPI_CLKDIV`, even though nearly every board header includes it.

<https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-sdk.pdf#config_parameters>