Rename the file org.wso2.analytics.solutions.is.analytics.widgets.average.session-1.0.63-javadoc.jar to org.wso2.analytics.solutions.is.analytics.widgets.average.session-1.0.63-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder org.wso2.analytics.solutions.is.analytics.widgets.average.session-1.0.63-javadoc
Double click index.html will open the index page on your default web browser.