doc: eliminate clutter in adc and gpio periph docs

cpu/cc26x0/include/periph_cpu.h

cpu/ezr32wg/include/periph_cpu.h

cpu/kinetis_common/include/periph_cpu.h

cpu/lm4f120/include/periph_cpu.h

cpu/lpc11u34/include/periph_cpu.h

cpu/lpc2387/include/periph_cpu.h

cpu/msp430fxyz/include/periph_cpu.h

cpu/nrf5x_common/include/periph_cpu.h

cpu/sam21_common/include/periph_cpu_common.h

cpu/sam3/include/periph_cpu.h

cpu/samd21/include/periph_cpu.h

cpu/saml21/include/periph_cpu.h

cpu/stm32f0/include/periph_cpu.h

cpu/stm32f1/include/periph_cpu.h

cpu/stm32f3/include/periph_cpu.h

cpu/stm32f4/include/periph_cpu.h

cpu/stm32l1/include/periph_cpu.h