doc: building: mention Zephyr USB device_next support in a note

Move the mention of experimental support for the Zephyr USB device "next"
stack to a note.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>

doc/building.rst

@@ -33,12 +33,14 @@ To use the ``release`` configuration, which has reduced log levels, set ``FILE_S
 
    west build -b lpcxpresso55s16/lpc55s16 cannectivity/app/ -- -DFILE_SUFFIX=release
 
-Build configurations for using the experimental ``device_next`` USB device stack in Zephyr are also
-provided. These can be selected by setting either ``FILE_SUFFIX=usbd_next`` or
-``FILE_SUFFIX=usbd_next_release``.
-
 After building, the firmware can be flashed to the board by running the ``west flash`` command.
 
+.. note::
+
+   Build configurations for using the experimental ``device_next`` USB device stack in Zephyr are
+   also provided. These can be selected by setting either ``FILE_SUFFIX=usbd_next`` or
+   ``FILE_SUFFIX=usbd_next_release``.
+
 USB Device Firmware Upgrade (DFU) Mode
 --------------------------------------