doc/driver-guide.md: Update doc on return value

Change driver guide to expect drivers to return negative errno codes instead of
enums for error codes. Those have the following advantages:

- More efficient implementation of POSIX compatibility wrappers:
    - In case of error, just an `errno = -retval; return -1` is needed
- Better interoperability
    - A lot of code within RIOT and in external code is doing so already.
      Applying this consistently allows passing error codes through, rather
      than translating between errno codes and custom enum values
- Not reinventing the wheel
    - Defining and documenting custom error codes for each driver is unneeded
      effort
    - The error classes that device driver have to report are largely the same.
      There is no reason to have dozens of error codes for an input/output
      error, each with a unique spelling a programmer needs to memorize

doc/doxygen/src/driver-guide.md

@@ -250,7 +250,7 @@ For more detailed information on how the signature of the init functions should
 look like, please refer below to the specific requirements for network devices
 and sensors.
 
-## Return values                                  {#driver-guide-return-values}
+## Return Values                                   {#driver-guide-return-values}
 
 As stated above, we check communication of a device during initialization and
 handle error return values from the lower layers, where they exist. To prevent
@@ -266,18 +266,49 @@ int16_t tmpabc_read(const tmpabc_t *dev)
 }
 @endcode
 
-Whenever you implement status/error return values in your driver, they
-should be named, meaning that the driver MUST define an enum assigning names to
-the actual used value, e.g.
+Whenever you implement status/error return values in your driver, magic numbers
+MUST ***NOT*** be used. Instead, use negative `errno` codes (such as `-EIO` for
+an input/output error) and *document* every single error code that can be
+return for each function using the `reval` Doxygen command. E.g. like this:
 
-@code{.c}
-enum {
-    TMPABC_OK    = 0,       /**< all went as expected */
-    TMPABC_NOI2C = -1,      /**< error using the I2C bus */
-    TMPABC_NODEV = -2       /**< no device with the configured address found on the bus */
-};
+@code
+/**
+ * @brief               Initialize the foo device and its descriptor
+ * @param[out]  dev     Device descriptor to initialized
+ * @param[in]   params  Device initialization parameters
+ *
+ * @retval      0       Success
+ * @retval      -ENXIO  No foo device connected to the I2C bus
+ * @retval      -ENODEV Device using the foo I2C address is not a foo device
+ * etc.
+ */
+int foo_init(foo_t *dev, const foo_params_t *params);
 @endcode
 
+You can multiplex this information whenever this is reasonable, as long as a
+negative return value indicates an error without exceptions. E.g. like this:
+
+@code
+/**
+ * @brief               Perform a relative humidity measurement
+ * @param[in]   dev     Humidity sensor to perform the measurement on
+ *
+ * @return              The relative humidity in percent
+ * @retval      -EIO    Communication with the sensor failed
+ */
+int foo_humidity(const foo_t *dev);
+@endcode
+
+### Documenting Return Values                  {#driver-guide-return-values-doc}
+
+With the exception of functions returning `void`, all return values have to be
+documented. Use the `return` Doxygen command to describe what is returned. In
+order to document special return value (such as error codes), use the `retval`
+command. The `retval` command expects the specific value to document as first
+argument (no spaces in the value allowed!), and a description (spaces allowed
+here) as second. It is safe to use both `return` and `retval` commands, or just
+one of them - whatever makes most sense for a given function.
+
 ## General Device Driver Checklist            {#driver-guide-general-checklist}
 
 - *MUST*: the supported feature set and any custom behavior is clearly