Do you just want your games installed there, or do you want the whole Steam installation?
If it's the whole installation, uninstall Steam and reinstall in under your Documents folder (backup data accordingly.) If you just want your games there, set your Documents folder as the install location under downloads. You can move the contents of the steamapps/common folder over, restart Steam and it will find the files again. (if it doesn't, install the game again, making sure the install directory is Documents, and it will then find the existing files.)