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

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

docs/library/pyb.ADC.rst

+.. currentmodule:: pyb
 .. _pyb.ADC:
 
 class ADC -- analog to digital conversion

docs/library/pyb.Accel.rst

+.. currentmodule:: pyb
+
 class Accel -- accelerometer control
 ====================================
 

docs/library/pyb.CAN.rst

+.. currentmodule:: pyb
+
 class CAN -- controller area network communication bus
 ======================================================
 

docs/library/pyb.DAC.rst

+.. currentmodule:: pyb
 .. _pyb.DAC:
 
 class DAC -- digital to analog conversion

docs/library/pyb.ExtInt.rst

+.. currentmodule:: pyb
 .. _pyb.ExtInt:
 
 class ExtInt -- configure I/O pins to interrupt on external events

docs/library/pyb.I2C.rst

+.. currentmodule:: pyb
 .. _pyb.I2C:
 
 class I2C -- a two-wire serial protocol

docs/library/pyb.LCD.rst

+.. currentmodule:: pyb
+
 class LCD -- LCD control for the LCD touch-sensor pyskin
 ========================================================
 

docs/library/pyb.LED.rst

+.. currentmodule:: pyb
 .. _pyb.LED:
 
 class LED -- LED object

docs/library/pyb.Pin.rst

+.. currentmodule:: pyb
 .. _pyb.Pin:
 
 class Pin -- control I/O pins

docs/library/pyb.RTC.rst

+.. currentmodule:: pyb
 .. _pyb.RTC:
 
 class RTC -- real time clock

docs/library/pyb.SPI.rst

+.. currentmodule:: pyb
 .. _pyb.SPI:
 
 class SPI -- a master-driven serial protocol

docs/library/pyb.Servo.rst

+.. currentmodule:: pyb
 .. _pyb.Servo:
 
 class Servo -- 3-wire hobby servo driver

docs/library/pyb.Switch.rst

+.. currentmodule:: pyb
+
 class Switch -- switch object
 =============================
 

docs/library/pyb.Timer.rst

+.. currentmodule:: pyb
 .. _pyb.Timer:
 
 class Timer -- control internal timers

docs/library/pyb.UART.rst

+.. currentmodule:: pyb
 .. _pyb.UART:
 
 class UART -- duplex serial communication bus

docs/library/pyb.USB_VCP.rst

+.. currentmodule:: pyb
+
 class USB_VCP -- USB virtual comm port
 ======================================