Require all words in multi-term online documentation searches

Make multi-word searches in the online documentation require every entered word to occur within the same result topic. Adding a word should narrow the results rather than returning every topic that contains any one of the words.

For example, searching for button pressed should return sections that contain both button and pressed, not unrelated sections that mention only buttons. A topic is one indexed documentation section: a heading and the content belonging to that heading.

If strict matching finds nothing, the interface may offer an explicit way to broaden the search to topics containing any word, without changing the predictable all-words default.