Log Message:
gtk-doc: update to 1.28.

GTK-Doc 1.28  (Mar 24 2018)
==============

Bug fixes.

 o 791928 : gtk-doc doesn't understand 'stability: Obsolete'
 o 792148 : cmake: Installed GtkDocConfig.cmake has incorrect bindir path
 o 792661 : use pygments to do syntax highlighing
 o 793599 : Please run testsuite with V=0
 o 794051 : rebase: fix on-line location extraction from devhelp files
 o 794182 : private_header regex is broken