Add note to rom_pick_ab_partition to see rom_pick_ab_update_partition when in a flash update boot

Author: will-v-pi

src/rp2_common/pico_bootrom/include/pico/bootrom.h

@@ -704,6 +704,9 @@ static inline int rom_load_partition_table(uint8_t *workarea_base, uint32_t work
  * 
  * NOTE: This method does not look at owner partitions, only the A partition passed and it's corresponding B partition.
  * 
+ * NOTE: You should not call this method directly when performing a Flash Update Boot before calling `explicit_buy`, as it may prevent
+ * any version downgrade from occuring - instead \see rom_pick_ab_update_partition() which wraps this function.
+ * 
  * \param workarea_base base address of work area
  * \param workarea_size size of work area
  * \param partition_a_num the A partition of the pair