This currently isn't supported. Out of curiosity, just how large do your descriptions get? Naturally, the completion description window allows for an arbitrarily-long HTML description, which can include info on not just the completion, but any parameters it takes, but the parameter description tool tip is currently designed to have either no description, or a one-liner.
If you'd like to see this feature, please add a Feature Request on RSyntaxTextArea's SourceForge page
. Or, you can dig into ParameterizedComlpletionDescriptionToolTip.java yourself; I'll gladly take patches!
One concern I'd have is that if the descriptions aren't forced to be short, the positioning of the tip becomes problematic. For example, if one parameter has a 10-line description, but another has a 2-line description, that difference in "height" of the tool tip might cause it to be displayed below the caret for the first parameter, but above the caret for the second one (for example, if the editor was near the top of your monitor). The (possible) large variation in tip window size between parameters might be visually jarring to users. Although it may not be as bad as I think.