As far as I understand you are injecting the repository through the constructor of Document Service as ducumentMockRepository.
So you can setup this mock with any expectations you want.
For your case you've to substitute DbContext by mock as well
// I hope you have an interface to abstract DbContext?
var dbContextMock = MockRepository.GenerateMock<IDbContext>();
// setup expectations for DbContext mock
dbContextMock.Expect(...)
// bind mock of the DbContext to property of repository.DbContext
ducumentMockRepository.Expect(mock => mock.DbContext)
.Return(dbContextMock)
.Repeat()
.Any();