I discovered how to specify a javadoc URL for external libraries being used in my project, and having set that up, Shift+F1 works nicely. I can see the URL recorded in a JAVADOC node in the appropriate XML file under the libraries folder.
Unfortunately, when there is a gradle sync, the setting is lost which is very frustrating. The XML file seems to be over-written.
The gradle configuration is using the 'idea' gradle plugin if that's relevant...
Anyone know how I can stop this from happening?