doc: man-page for size command

Provide a man-page for the size command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/usage/index.rst

@@ -38,6 +38,7 @@ Shell commands
    pstore
    qfw
    sbi
+   size
    true
    scp03
    reset

doc/usage/size.rst

@@ -0,0 +1,40 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+size command
+============
+
+Synopsis
+--------
+
+::
+
+    size <interface> <dev[:part]> <filename>
+
+Description
+-----------
+
+The size command determines the size of a file and sets the environment variable
+filesize to this value. If filename points to a directory, the value is set to
+zero.
+
+If the command fails, the filesize environment variable is not changed.
+
+dev
+    device number
+
+part
+    partition number, defaults to 1
+
+filename
+    path to file
+
+Configuration
+-------------
+
+The size command is only available if CONFIG_CMD_FS_GENERIC=y.
+
+Return value
+------------
+
+The return value $? is set to 0 (true) if the command succeded and to 1 (false)
+otherwise.