kernel: i2c-comm: Explain why the I2C bus needs pull-ups

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

labs/kernel-i2c-communication/kernel-i2c-communication.tex

@@ -136,8 +136,11 @@ \section{Add pinctrl properties to the Device Tree}
 \item \ksym{MUX_MODE2} corresponds to muxing mode 2, as explained in the
       datasheet.
 \item \ksym{PIN_INPUT_PULLUP} puts the pin in pull-up mode (remember
-      that our pins support both pull-up and pull-down). It seems to
-      be needed for I2C bus operation.
+      that our pins support both pull-up and pull-down). By design, an
+      I2C line is never actively driven high, devices either pull the
+      line low or let it floating. As we plug our device directly on the
+      bus without more analog electronics, we need to enable the
+      internal pull-up.
 \end{itemize}
 
 Now that pin muxing settings have been explained, edit your board