Documentation/rp2040: document fastboot_usb config for waveshare boards

Add a "fastboot_usb" section to the waveshare-rp2040-zero and
waveshare-rp2040-lcd-1.28 board doc pages, describing the USB fastboot
composite configuration added by the previous commits: fastbootd runs
on boot instead of NSH, composed together with CDC/ACM for the
console, and reachable on the host via fastboot devices/getvar/reboot.

Assisted-by: OpenCode:claude-sonnet-5
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>
diff --git a/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-lcd-1.28/index.rst b/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-lcd-1.28/index.rst
index 0aa5c7c..b851324 100644
--- a/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-lcd-1.28/index.rst
+++ b/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-lcd-1.28/index.rst
@@ -88,6 +88,17 @@
 CDC/ACM with MSC USB composite driver. ``conn`` command enables the composite
 device.
 
+fastboot_usb
+------------
+
+Runs the USB fastboot daemon (``fastbootd``) on boot instead of NSH, exposed
+as a USB composite device together with CDC/ACM (console enabled in USB
+Port, at 115200 bps). On the host, ``fastboot devices``, ``fastboot getvar
+<var>``, and ``fastboot reboot``/``fastboot reboot bootloader`` are
+available. More details about the fastboot daemon are available at
+`fastbootd — NuttX latest documentation
+<https://nuttx.apache.org/docs/latest/applications/system/fastboot/index.html>`_.
+
 nsh
 ---
 
diff --git a/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-zero/index.rst b/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-zero/index.rst
index e2d3ae6..8fc5e55 100644
--- a/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-zero/index.rst
+++ b/Documentation/platforms/arm/rp2040/boards/waveshare-rp2040-zero/index.rst
@@ -98,6 +98,17 @@
 
    $ ./tools/configure.sh waveshare-rp2040-zero:<configname>
 
+fastboot_usb
+------------
+
+Runs the USB fastboot daemon (``fastbootd``) on boot instead of NSH, exposed
+as a USB composite device together with CDC/ACM (console enabled in USB
+Port, at 115200 bps). On the host, ``fastboot devices``, ``fastboot getvar
+<var>``, and ``fastboot reboot``/``fastboot reboot bootloader`` are
+available. More details about the fastboot daemon are available at
+`fastbootd — NuttX latest documentation
+<https://nuttx.apache.org/docs/latest/applications/system/fastboot/index.html>`_.
+
 gpio
 --------