There's a difference between documentation for w/e you are working on and documentation for this cool new framework you saw on HN that has little to do with your duties.
But I have e.g. said to my boss "I'm reading up on 'Orrible for the next two hours", put on the headphones (Metal Machine Music by preference ... it never gets old) and read far too much about Oracle, ignoring with explicit permission anyone who interrupts. That's clear. New thing you've never seen before that is suddenly your problem? TOTALLY hit the books on the clock.
Learning about the thing you are dealing with at work is totally work, and if it makes your work sufficiently more effective then not doing it is the wrong thing.