That's exactly what I did in a recent project. Basically, I found a cool library, but the docs were pretty spotty, so I fleshed it out a bit as I became more familiar with the code. I ended up doing some minor refactoring to make the library more consistent (e.g. naming, this was just before a major release).

After that, I ended up finding a bug, and since I was already familiar with the code from fixing minor documentation stuff, I was able to submit a patch without asking anyone.