Rename the file discovery-plugin-strategy-starter-sentinel-limiter-6.21.0-javadoc.jar to discovery-plugin-strategy-starter-sentinel-limiter-6.21.0-javadoc.zip
Use your favourite unzip tool (WinRAR / WinZIP) to extract it, now you have a folder discovery-plugin-strategy-starter-sentinel-limiter-6.21.0-javadoc
Double click index.html will open the index page on your default web browser.