Warsaw — Former Solidarity leader Lech Walesa was detained by Polish police - the second time in six days. They stopped Mr. Walesa, the Rev. Henryk Jankowski, and another man as the three were driving from Gdansk to Warsaw. The police apparently wanted to prevent Mr. Walesa from attending ceremonies commemorating the World War II Jewish ghetto uprising. Government spokesmen said Walesa was not formally arrested, but did not explain why he had been detained.