You really have two different issues. You want the documents folder back in your home directory, and you want the documents folder to appear in the side bar. It's easy to do both.
Open a Finder window. Select desktop from the side-bar. Or if Desktop does not appear in the sidebar simply view your actual desktop. (I can't remember if desktop is in the side-bar by default or if I added it.)
Drag the Documents folder from your desktop to your home directory. Your home directory will be listed on the side-bar as your user name with a picture of a house next to it.
Next, go to your home directory, and drag the documents folder to the side bar, and it will be added.
Good luck! Let us know if you need more help with this.
EDIT: When you add the documents folder to the side bar, look for the visual cues the Finder gives you. If you drag the documents folder over a sidebar item (and that item becomes highlighted in blue) and release the folder, the folder will be moved to that side bar location (for instance the Applications folder). You need to drag the documents folder to the side bar and position it in between two items already there, when you do so a blue line with a circle on the right hand side will show up showing that you are going to insert it there. When you see that line, you can let go and the folder will stay in the sidebar.