docs/machine.*: Add "currentmodule:: machine" directive.

Makes sure that classes described in these separate files are properly
designated as belonging to "machine" module in indexes.

docs/library/machine.ADC.rst

+.. currentmodule:: machine
 .. _machine.ADC:
 
 class ADC -- analog to digital conversion

docs/library/machine.I2C.rst

+.. currentmodule:: machine
 .. _machine.I2C:
 
 class I2C -- a two-wire serial protocol

docs/library/machine.Pin.rst

+.. currentmodule:: machine
 .. _machine.Pin:
 
 class Pin -- control I/O pins

docs/library/machine.RTC.rst

+.. currentmodule:: machine
 .. _machine.RTC:
 
 class RTC -- real time clock

docs/library/machine.SD.rst

+.. currentmodule:: machine
 .. _machine.SD:
 
 class SD -- secure digital memory card

docs/library/machine.SPI.rst

+.. currentmodule:: machine
 .. _machine.SPI:
 
 class SPI -- a master-driven serial protocol

docs/library/machine.Timer.rst

+.. currentmodule:: machine
 .. _machine.Timer:
 
 class Timer -- control internal timers

docs/library/machine.UART.rst

+.. currentmodule:: machine
 .. _machine.UART:
 
 class UART -- duplex serial communication bus

docs/library/machine.WDT.rst

+.. currentmodule:: machine
 .. _machine.WDT:
 
 class WDT -- watchdog timer