doc: describe the false command

Provide a man-page for the false command.

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

doc/usage/false.rst

@@ -0,0 +1,28 @@
+false command
+=============
+
+Synopsis
+--------
+
+::
+
+    false
+
+Description
+-----------
+
+The false command sets the return value $? to 1 (false).
+
+Example
+-------
+
+::
+
+    => false; echo $?
+    1
+    =>
+
+Configuration
+-------------
+
+The false command is only available if CONFIG_HUSH_PARSER=y.

doc/usage/index.rst

@@ -19,6 +19,7 @@ Shell commands
    button
    echo
    exit
+   false
    for
    loady
    mbr