Make it more obvious that XPDF headers have unstable API/ABI

Status Job ID Name Coverage
  Build
passed #64319
build

00:05:17

passed #64324
build_android

00:02:18

passed #64320
build_clang_libcpp

00:05:13

passed #64323
build_clazy

00:04:45

passed #64321
build_gcc_49

00:07:49

passed #64322
build_mingw64_fedora

00:03:39