RPI px4_no_shield.config: add documentation for connected HW

posix-configs/rpi/px4_no_shield.config

+# config for a quad without any shield:
+# - u-blox GPS connected via UART
+# - PWM output using PCA9685 via I2C
+# - RC input via shared memory from ppmdecode (https://github.com/crossa/raspberry-pi-ppm-rc-in),
+#   GPIO input
 uorb start
 param load
 param set SYS_AUTOSTART 4001