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();