glib: Documentation fixes for viewer preferences

Clarify documentation about returned values
of recently added viewer preferences.

poppler/poppler#290
6 jobs for glib-viewer-preferences in 17 minutes and 28 seconds (queued for 5 minutes and 18 seconds)
Status Job ID Name Coverage
  Build
passed #521093
build

00:03:30

passed #521098
build_android

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passed #521094
build_clang_libcpp

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passed #521097
build_clazy

00:07:41

passed #521096
build_mingw64_fedora

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passed #521095
build_ubuntu_16_04

00:03:41