document: Handle UTF16-LE annotations
I can produce such annotations when adding annotations to a PDF attachement from the standard mail app on my iPhone (iOS 12.1). They currently all show as "ÿþÚ" rather than the actual string content.
UTF16-BE vs UTF16-LE is detected by inferring the endianness from the first two bytes of the string (0xFF 0xFE and 0xFE 0xFF aka Byte Order Marker).