In IntelliJ Idea I set offline javadoc of Java SDK in Project structure -> SDKs -> Documentation Paths and add path to documentation folder. Whenever I want to see tooltip I just press Ctrl + Q to see offline documentation for a symbol at caret. But it works only for Java SDK. For example - I am using servlet-api.jar library, I downloaded servlet-api javadoc, but I cannot import it to IntelliJ Idea in any way. I tried to add it to SDKs -> Documentation Paths but it didn't work. Is it a way how to import offline javadoc of other libraries to see tooltips easily?
When the project type is not Maven or Gradle, adding Javadoc for particular library is done via settings in Project Structure(Ctrl + Shift + Alt + S). Add library by choosing Java from option. Then add particular JavaDoc for that library. Then choose apply. To see tooltip window not only by pressing Ctrl + Q but also by hover over symbol, choose File -> Settings -> Editor -> General -> and check Show quick documentation on mouse move.