<p>NDoc has an XML element <strong>inheritdoc</strong> which allows you to inherit documentation of a member from the parent class (or an implemented interface). However, Visual Studio (i.e. the C# compiler) does not understand this tag and complains about the documentation not being present or complete. So does StyleCop and some other tools. Is there an alternative approach? How do you go about keeping the docs complete, yet without duplicating the XML descriptions?</p>

One alternative is to use
GhostDoc
- an add-in for Visual Studio that automatically generates comments for you. This duplicates the XML description of course, which is part of what you're trying to avoid - but at least it does it automatically for you.
What happens if you just leave off the docs entirely for methods which are being inherited, or overriding interface methods? I suspect it depends on how you've got NDoc configured, but certainly in MSDN documentation seems to just naturally inherit the docs - and a quick check
suggests
that VS won't whinge when you don't prodive docs for an inherited method. Worth a try, certainly.