Enhance I2C bus initialization and clarify bus capacitance

• Added the missing implementation of inputs::initializeI2C().
  Surprisingly, no sketch is using it so it went unnoticed.
• Now the internal pull-up resistors in the I2C bus can be disabled thanks to another inputs::initializeI2C() parameter.
• Additional advice and explanations are given regarding pull-up resistors in the I2C bus.

Full Changelog: 7.6.0...7.7.0