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).
6 jobs for utf16le in 3 minutes and 34 seconds
Status Job ID Name Coverage
  Build
passed #200488
build

00:03:33

passed #200493
build_android

00:01:02

passed #200489
build_clang_libcpp

00:03:24

passed #200492
build_clazy

00:03:33

passed #200491
build_mingw64_fedora

00:03:14

passed #200490
build_ubuntu_16_04

00:01:46