WASHINGTON — Whatever larger themes are sounded when the United States Holocaust Memorial Museum commemorates its 20th anniversary here this weekend, whatever is said at a Monday ceremony by former President Bill Clinton or by the museum’s founding chairman, Elie Wiesel, and whatever assessments are made about its influence, accomplishments or limitations, it will take a visit to its new exhibition, “Some Were Neighbors,” to grasp one aspect of this imposing institution’s power. It reveals the demonic not in grand forces, but in the most minute details.