Ok sorry, terminology put me off. Make sure you've read the documentation (http://doc.silverstripe.org/framework/en/trunk/misc/collaboration-on-git) and I'll try and simplify it.
So you first fork the module which you can then clone to your machine (e.g I would do git clone firstname.lastname@example.org:willrossi/sapphire.git).
3.0 is currently using the 'master' branch. Most of your changes should be on the master branch. So ensure that is checked out
git checkout master
But you don't want to work directly on master as you'll then need to do a lot of merges so lets branch out for our new feature
git checkout -b docupdates
Now you can make your changes, git add, and git commit each change and make sure you use the code 'MINOR: ' on any documentation updates.
git commit -a -m "MINOR: update documentation .. "
After you're happy with that piece of work and you want to pull your changes to your fork we created in #1. To make our life easier we'll push our branch we created ('docupdates') to the remote.
git push origin docupdates:docupdates.
Now you can make your pull request...
http://doc.silverstripe.org/framework/en/trunk/misc/collaboration-on-git has a lot more information but happy to provide a video explaination if that would help.
For documentation updates you can also edit the docs directly through the github interface by clicking the 'Edit this file' button in the Github UI.