bin: document %% for ltldo and ltlcross

I had no idea this was working...
Reported by Joachim Klein.

* src/bin/common_trans.cc: Document it.
* src/tests/ltlcross3.test: Test it.

src/tests/ltlcross3.test

@@ -161,6 +161,11 @@ test 5 = `wc -l < out.csv`
 check_csv out.csv
 
 
-# Diagnose empty automata
-run 1 ../../bin/ltlcross ': %f >%O' -f a 2>stderr
+# Diagnose empty automata, and make sure %% is correctly replaced by %
+run 1 ../../bin/ltlcross ': %f >%O; echo %%>foo' -f a 2>stderr
 test 2 = `grep -c 'error: empty output.' stderr`
+cat foo
+cat >expected<<EOF
+%
+EOF
+diff foo expected