Documentation: add '-f' description for umount command

Reference:
https://github.com/apache/nuttx-apps/pull/3240

Signed-off-by: zhengyu16 <zhengyu16@xiaomi.com>

Documentation/applications/nsh/commands.rst

@@ -1725,12 +1725,18 @@ reads as zero bytes.
 
 **Command Syntax**::
 
-  umount <dir-path>
+  umount [-f] <dir-path>
 
 **Synopsis**. Un-mount the file system at mount point ``<dir-path>``.
 The ``umount`` command can only be used to un-mount volumes previously
 mounted using :ref:`mount <cmdmount>` command.
 
+**Options**
+
+======  ======================================================
+``-f``  Force unmounting of the filesystem even if it is busy.
+======  ======================================================
+
 **Example**::
 
   nsh> ls /mnt/fs