doc: describe the false command

Provide a man-page for the false command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/false.rst b/doc/usage/false.rst
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+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.