Typically Yes That k-base document will resolve, OR at least help....
This is how you see if it is your computer software causing the issue OR if it is the battery in your unit.
attempt to restore the device, when you recieve error 21 (unable to restore "device" unknown error occured 21)
make sure your device has been plugged in long enough to recieve some sort of charge. ( computer, dock, charger makes no difference)
Unplug the unit.....
Did it turn itself off and act like a brick? If Yes, then your battery itself is faulty and needs replaced!
iphone 4 owners may purchase a replacement from amazon for around 5 dollars and do the replacement them selfs ( walkthrough on youtube) "how to replace battery on iphone 4"
other devices, same behavior, depending on your modle of IOS device you have may dictate if you can repair this or IF you need to take to apple service provider / or call apple care 1-800-APL-CARE.
If your device is less than a year old OR if you have an apple care protection plan ( AND the unit has not became wet) you may send it in to apple for a battery replacement. ( if it had been damaged depending on the damage ther may be a fee. If it is outside of warrenty generally 79$ {iphone} is the pricing you are looking at if the unit is outside of coverage.)
Hope this helps!!! I was able to solve my error 21 by simply swapping out the battery THEN restoring the unit via itunes without further errors!
Cheers!