If you look at the source code you'll see that this method:
- clears the buffer arrays for primitive types (to the default value)
- clears object buffers to null (so that objects can be garbage collected)
- does not release or shrink the buffers themselves (so that arrays are still kept and referenced).
So "Does not release internal buffers." talks about the last bullet (if you have a large collection then it won't release the buffer arrays; may be worth to discard the entire object instead of clearing it).