So some of the -reference ports that install multiple docs or "books" can
make sure they get installed correctly. This impacts the -reference ports
of glib20, gtk20 and gtk30.
The problem was that for example in glib20 the reference was installed in
share/doc/glib/{glib,gio,gobject} however devhelp only look in
share/doc/${book} for the *.devhelp2 file.
PR: ports/173577
Submitted by: Victor Balada Diaz <victor@bsdes.net>
c0e23506ba