Add notes on TinyUSB documentation and quirks (#659)

Fixes #648

Author: maxgerhardt

docs/usb.rst

@@ -46,3 +46,61 @@ Also, this stack requires sketches to manually call
 sketch upload from the IDE.  If a sketch is run without this command
 in ``setup()``, the user will need to use the standard "hold BOOTSEL
 and plug in USB" method to enter program upload mode.
+
+Adafruit TinyUSB Configuration and Quirks
+-----------------------------------------
+
+The Adafruit TinyUSB's configuration header for RP2040 devices is stored
+in ``libraries/Adafruit_TinyUSB_Arduino/src/arduino/ports/rp2040/tusb_config_rp2040.h`` (`here <https://github.com/adafruit/Adafruit_TinyUSB_Arduino/blob/master/src/arduino/ports/rp2040/tusb_config_rp2040.h>`__).
+
+In some cases it is important to know what TinyUSB is configured with. For example, by having set 
+
+.. code:: cpp
+
+    #define CFG_TUD_CDC 1
+    #define CFG_TUD_MSC 1
+    #define CFG_TUD_HID 1
+    #define CFG_TUD_MIDI 1
+    #define CFG_TUD_VENDOR 1
+
+this configuration file defines the maximum number of USB CDC (serial)
+devices as 1. Hence, the example sketch `cdc_multi.ino <https://github.com/adafruit/Adafruit_TinyUSB_Arduino/blob/master/examples/CDC/cdc_multi/cdc_multi.ino>`__
+that is delivered with the library will not work, it will only create one
+USB CDC device instead of two. It will however work when the above
+``CFG_TUD_CDC`` macro is defined to 2 instead of 1.
+
+To do such a modification when using the Arduino IDE, the file can be
+locally modified in the Arduino core's package files. The base path can
+be found per `this article <https://support.arduino.cc/hc/en-us/articles/360018448279-Open-the-Arduino15-folder>`__,
+then navigate further to the ``packages/rp2040/hardware/rp2040/<core version>/libraries/Adafruit_TinyUSB_Arduino``
+folder to find the Adafruit TinyUSB library.
+
+When using PlatformIO, one can also make use of the feature that TinyUSB
+allows redirecting the configuration file to another one if a certain
+macro is set.
+
+.. code:: cpp
+
+    #ifdef CFG_TUSB_CONFIG_FILE
+        #include CFG_TUSB_CONFIG_FILE
+    #else
+        #include "tusb_config.h"
+    #endif
+
+And as such, in the ``platformio.ini`` of the project, one can add
+
+.. code:: ini
+
+    build_flags =
+      -DUSE_TINYUSB 
+      -DCFG_TUSB_CONFIG_FILE=\"custom_tusb_config.h\"
+      -Iinclude/ 
+
+and further add create the file ``include/custom_tusb_config.h`` as a copy
+of the original ``tusb_config_rp2040.h`` but with the needed modifications.
+
+*Note:* Some configuration file changes have no effect because upper levels
+of the library don't properly support them. In particular, even though the
+maximum number of HID devices can be set to 2, and two ``Adafruit_USBD_HID``
+can be created, it will not cause two HID devices to actually show up, because
+of `code limitations <https://github.com/adafruit/Adafruit_TinyUSB_Arduino/blob/7264c1492a73d9a285512752b03f2550841c06bc/src/arduino/hid/Adafruit_USBD_HID.cpp#L36-L37>`__.