Vendors can decide whether to provide support for a plugin or not. You have to accept the plugin as-is in the case of unsupported plugins.
That said, for a plugin with PlantUML support, the main documentation is probably the PlantUML docs, http://plantuml.com/guide. The plugins main function is probably to provide a text area for you to paste in your PlantUML text, with some options. It's unlikely to be complex to operate.
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