@Paul's fstab example is, basically, the way to go if you want an answer to the question as asked. However, I would suggest that unless you have specific reasons for only moving your documents, it might not be a bad idea to move the whole
/home directory to the new disk, and use the same technique to bind /home to its new location (or even just move /home/* to the root of a partition on the new drive and mount that partition at /home). This will provide two major benefits:
- You are likely to get more space for your home directory.
- You completely separate the operating system, system-wide configuration files and application binaries from your own files.
The point on separation to me is the bigger issue: it means that if you need to reinstall the OS for some reason, you won't have to worry anywhere near as much about your personal files (e-mails, documents, music, etc etc).
Unix mount points are a very flexible construct - do take advantage of that.