Docs: larify pin assignments for i2c (#173)

Author: skybrian

docs/wire.rst

@@ -1,18 +1,24 @@
 Wire (I2C Master and Slave)
 ===========================
 
-The RP2040 has two I2C devices, ``i2c0 (Wire)`` and ``i2c1 (Wire1)``
+The RP2040 has two I2C devices, ``i2c0 (Wire)`` and ``i2c1 (Wire1)``.
 
-The current implementation is compatible with the Arduino standard
-with the addition of two calls to allof the `SDA` and `SCL` pins
-to be set **before calling Wire.begin()**
+The default pins for `Wire` and `Wire1` vary depending on which board you're using.
+(Here are the pinout diagrams for `Pico <https://datasheets.raspberrypi.org/pico/Pico-R3-A4-Pinout.pdf>`_
+and `Adafruit Feather <https://learn.adafruit.com/assets/100740>`_.)
+
+You may change these pins **before calling Wire.begin() or Wire1.begin()** using:
 
 .. code:: cpp
 
         bool setSDA(pin_size_t sda);
         bool setSCL(pin_size_t scl);
 
-Both master and slave operation is supported.
+Be sure to use pins labeled ``I2C0`` for Wire and ``I2C1`` for Wire1 on the pinout
+diagram for your board, or it won't work.
+
+Other than that, the API is compatible with the Arduino standard.
+Both master and slave operation are supported.
 
 Master transmissions are buffered (up to 128 bytes) and only performed
 on ``endTransmission``, as is standard with modern Arduino Wire implementations.