Add some missing i2c function documentation

Add i2c_get_index for consistency - just calls i2c_hw_index

src/rp2_common/hardware_i2c/include/hardware/i2c.h

@@ -148,11 +148,33 @@ static inline uint i2c_hw_index(i2c_inst_t *i2c) {
     return i2c == i2c1 ? 1 : 0;
 }
 
+/*! \brief Convert I2C instance to hardware instance number
+ *  \ingroup hardware_i2c
+ *
+ * \param i2c I2C instance
+ * \return Number of I2C, 0 or 1.
+ */
+static inline uint i2c_get_index(i2c_inst_t *i2c) {
+    return i2c_hw_index(i2c);
+}
+
+/*! \brief Return pointer to structure containing i2c hardware registers
+ *  \ingroup hardware_i2c
+ *
+ * \param i2c I2C instance
+ * \return pointer to \ref i2c_hw_t
+ */
 static inline i2c_hw_t *i2c_get_hw(i2c_inst_t *i2c) {
     i2c_hw_index(i2c); // check it is a hw i2c
     return i2c->hw;
 }
 
+/*! \brief Convert I2C hardware instance number to I2C instance
+ *  \ingroup hardware_i2c
+ *
+ * \param Number of I2C, 0 or 1
+ * \return I2C hardware instance
+ */
 static inline i2c_inst_t *i2c_get_instance(uint instance) {
     static_assert(NUM_I2CS == 2, "");
     invalid_params_if(I2C, instance >= NUM_I2CS);