In Agile environments, your approach to documenting architecture should be similar to the one about documenting requirements. First and foremost, consider who will be reading the materials you prepare to be sure you're describing the right things in the right way. Your documentation doesn't need to be a lengthy Word document. You can use presentations, wiki pages, single diagrams, or even recordings from meetings when someone describes the architecture.
What is important is to gather feedback on the documented architecture. Again, in the same way, as with the documented requirements, it's important to reiterate the documents with your stakeholders to know where to improve them. Even though this may seem like you're wasting time, if done properly, it should save you some time in terms of delivering the product. Good enough documentation should help newcomers to start being productive faster and guide more familiar stakeholders...
