Due to the nature of most Agile development, the details of a given piece of functionality can change over time (in fact, if this isn't true, you aren't very agile). Therefor, what is on a given card in your backlog is just a description of that point in time, making it ill-suited for long-term documentation. Further, documenting too much in cards can lead to a large amount of difficultly finding information later, even if it hasn't changed.
My suggestion is to never look to cards as documentation. If you need to create documentation, create it in a place more suited for your needs. It is also worth noting that while things like Sharepoint and Confluence or even traditional documentation systems can be helpful, many Agile teams turn to more purpose-build documentation that is more effective. For example, if I want to know how an API works, I'd rather have that in an executable test so that I can see how it works, try variations, and be alerted if the application drifts from expectations. If I want to document style guides, a system like Zeplin bridges the gap between design apps and code.