doc: usage: man-page for ums command

Provide a man-page for the ums command - USB Mass Storage.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

diff --git a/doc/usage/index.rst b/doc/usage/index.rst
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--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -44,3 +44,4 @@ Shell commands
    scp03
    size
    true
+   ums

diff --git a/doc/usage/ums.rst b/doc/usage/ums.rst
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@@ -0,0 +1,57 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+ums command
+===========
+
+Synopsis
+--------
+
+::
+
+    ums <dev> [<interface>] <devnum[:partnum]>
+
+Description
+-----------
+
+Use the USB Mass Storage class (also known as UMS) to make accessible an U-Boot
+block device (fully or with :ref:`U-Boot's partition syntax <partitions>`)
+to a USB host and to enable file transfers. U-Boot, the USB device, acts as a
+simple external hard drive plugged on the host USB port.
+
+This command "ums" stays in the USB's treatment loop until user enters Ctrl-C.
+
+dev
+    USB gadget device number
+
+interface
+    interface for accessing the block device (mmc, sata, scsi, usb, ....)
+    defaults is "mmc"
+
+devnum
+    device number for selected interface
+
+partnum
+    partition number or 0 to expose all partitions, defaults to 0
+
+Example
+-------
+
+::
+
+    => ums 0 mmc 0
+    => ums 0 usb 1:2
+
+Configuration
+-------------
+
+The ums command is only available if CONFIG_CMD_USB_MASS_STORAGE=y
+and depends on CONFIG_USB_USB_GADGET and CONFIG_BLK.
+
+Return value
+------------
+
+The return value $? is set to 0 (true) when the USB stack was successfully
+started and interrupted, with Ctrl-C or after USB cable issue (detection
+timeout or cable removal).
+
+If an error occurs, the return value $? is set to 1 (false).