docs/library: Document that sys.version_info returns a 3-tuple only.
docs/library: Document that sys.version_info returns a 3-tuple only.

See issue #4970.

bvernoux/micropython,tobbad/micropython,henriknelson/micropython,pozetroninc/micropython,selste/micropython,bvernoux/micropython,adafruit/circuitpython,pozetroninc/micropython,pozetroninc/micropython,MrSurly/micropython,trezor/micropython,tobbad/micropython,pramasoul/micropython,bvernoux/micropython,selste/micropython,selste/micropython,bvernoux/micropython,kerneltask/micropython,tobbad/micropython,MrSurly/micropython,tobbad/micropython,adafruit/circuitpython,trezor/micropython,kerneltask/micropython,kerneltask/micropython,adafruit/circuitpython,henriknelson/micropython,bvernoux/micropython,trezor/micropython,selste/micropython,pramasoul/micropython,adafruit/circuitpython,pramasoul/micropython,MrSurly/micropython,pozetroninc/micropython,henriknelson/micropython,pozetroninc/micropython,henriknelson/micropython,tobbad/micropython,adafruit/circuitpython,adafruit/circuitpython,MrSurly/micropython,kerneltask/micropython,henriknelson/micropython,trezor/micropython,MrSurly/micropython,pramasoul/micropython,pramasoul/micropython,kerneltask/micropython,selste/micropython,trezor/micropython