As I was writing and using my custom GhostDoc rules (here and here), I was thinking that it would be nice if GhostDoc was able to clear the existing documentation and then apply the DocumentThis command. I emailed Roland Weigelt about the feature idea not realising there is a forum for feature requests. He quickly replied saying that the feature is already there albeit undocumented. This is exactly what I was after.
If you open up the keyboard bindings and do a search for GhostDoc, you will find a RebuildDocumentation command.
I have assigned the RebuildDocumentation to a Ctrl+Shift+D, Ctrl+Shift+R combination and changed DocumentThis to Ctrl+Shift+D, Ctrl+Shift+D combination. Roland said to make sure that the new keyboard binding is only used in Text Editor (as highlighted above).