Short answer

Create and update documentation continuously - in sync with your sprints or iterations.

Longer answer

If you defer documentation to “later”, I am reasonably sure it might never be done. Don’t procrastinate - but make (brief, short, compact, economical) documentation a habit, like checking-in source code in version control or writing automated tests (you test, don’t you?).

In my opinion it’s way better to have short up-to-date and current documentation of only some aspects of the system, than to just have a plan to sometimes document the whole system…

Reserve a brief period of time every day (or at least every week) to catch up with your documentation tasks. Close your door, your email reader and your messenger apps, flip your phone to silent mode… and jot down some important, interesting, risky or otherwise valuable information about your current topics.