No, there's no standard documentation or summary for plugins. Most of them are hosted on Github which displays their readme files on the main page. Usually this will tell you enough about the plugin, but not always. If you'd like more info about something, consider opening an issue on the github repo and I'm sure that 99% of developers would happily oblige. In terms of finding the github page, if you have Package Control installed, you can use the "Discover Packages" command to go to the repo page of each.