... explaining obvious things would be an offense to the reader ...

In my opinion, the line for this is far far far further down than you would ever expect until you've been doing docs for quite some time.

Especially if you're writing to a relatively wide audience... if you can narrow your audience to only an advanced few, you can take shortcuts, but even then, advanced people know to skip the things they already know. Beginners cannot make up stuff that they don't know.