doc: exception command

Create man-page for exception command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index f75bd08..83cfbaf 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -19,6 +19,7 @@
    button
    conitrace
    echo
+   exception
    exit
    false
    for