If I have been denied a visitor visa to the UK previously, is it a good idea to add the refusal letter or any other supporting document in any new application for a Schengen visa to clear that point?
Since the UK is not part of the Schengen Area, I don't see how this would be necessary (and why you would want to do that anyway). If the country where you are applying for a Schengen visa thinks that they are interested in whether you have applied for other visa before, then they have internal ways to do that. They don't need to ask you.
There is the Schengen Information System (SIS) which in part handles such queries automatically. The UK as an EU member state participates in the SIS and exchanges information with other EU states, even though it is not part of Schengen (and therefore cannot grant Schengen visa).