It went wherever you configured the mount point for the partition when you installed. If you did not configure a mount point when you installed, then you will need to mount it now. You can use the Disk Utility or GParted to view the partitions on the disk, or
parted -l on the command line.
The first thing you need to do is choose where to mount the partition. You can mount it in a new directory in
/media, or you could use it for your
/home/yourname/Documents folder. If you already have files in your Documents folder, you will need to mount the partition somewhere else temporarily and move the files. If the partition in question is
/dev/sda2, then you can do this with:
sudo mount /dev/sda2 /mnt
sudo mv /home/yourname/Documents/* /mnt/
Look up the UUID of the partition with
sudo blkid, then edit /etc/fstab with
sudo gedit /etc/fstab. You will need to add a line like this:
UUID=theuuid /home/yourname/Documents/ ext4 defaults 0 0
Then reboot, and your Documents folder will be stored on that partition.