When invoking "Document Method", it doesn't check if an documentation header already exists or not, it just inserts a(nother) whole documentation header. Would be great if VA would offer to complete documentation (in case parameters or tags are missing), or just dont offer it.
The idea of updating an existing comment block from Document Method with new parameters did come up once before:
are you thinking of just adding extra parameters? Adding all parameters if VA doesn't detect parameter information in the comment block? In C#, if we assume XML comments then working out which parameters are commented is probably straight forward, but in C++ with much more random and unstructured comments, it becomes quite a bit harder.
For now, I'd be happy with C# support and structured XML comments. The IDE now actually offers to add missing parameter documentation if there is partial documentation (with sandcastle installed it seems), so maybe just checking for existing doc is already fine?