AST-1561 Improvements

Generally small changes that I gathered while working on the MSReader that I'd recommend applying.

Some changes are a bit larger as I found some limitations along the way. Particularly:

• ParameterSet.h: I think it is good if these comments explain a bit more what the 2nd parameter means because it is slightly counter-intuitive that a getter has a default return value. I added them to each so that on mouse hover, VSCode shows it immediately. I found myself digging into the code again for this.
• MultiMSReader.h: I found that the documentation at the top was just an exact copy of the (single) MSReader, meaning there is no documentation whatsoever about the MultiMSReader.h. I made a proposed fix for that. Also avoids 'code' (comments) duplication.