You can document your plug-in's configuration options and behavior by placing a TEXT or RICH-TEXT (rtf) formatted file in the plug-in directory. Give the file the same root name as your plug-in and an .rtf extension (myplug-in.exe uses myplug-in.rtf). Use the .rtf extension even if the file is plain-old-text. You can use Wordpad or OpenOffice to create RTF files. Of course, notepad can create text files. The built-in viewer does not support advanced features like embedded graphics. Stick to stylized text.
When the user is editing the signal's configuration options, they can press the help button to view your document. This is a good place to remind the user (or yourself) what the plug-in does, what the options do, or anything they should do or avoid ( 'be sure to select rising edge clock' or 'we ignore SYNC settings for now').