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/index.rst b/doc/usage/index.rst
index d3abf2d..0309abf 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -19,6 +19,7 @@
    button
    echo
    exit
+   false
    for
    loady
    mbr