Update obsolete nbtree array preprocessing comments.

Update obsolete nbtree array preprocessing comments.

The array->scan_key references fixed up at the end of preprocessing
start out as offsets into the arrayKeyData[] array (the array returned
by _bt_preprocess_array_keys at the start of preprocessing that involves
array scan keys).  Offsets into the arrayKeyData[] array are no longer
guaranteed to be valid offsets into our original scan->keyData[] input
scan key array, but comments describing the array->scan_key references
still talked about scan->keyData[].  Update those comments.

Oversight in commit b5249741.