gtkdochelper: Add 'overrides.txt' to the content files
gtkdochelper: Add 'overrides.txt' to the content files

If a `<modulename>-overrides.txt` file exists in the docs directory it
means it's intended to be used in place of the one gtk-doc generates.

GLib and GTK+, for instance, ship with one because some of the types
they provide — like the thread primitives, or the platform macros —
contain architecture-dependent fields that should not be accessed
directly.

This commit should close the last bit of issue #550.

pexip/meson,wberrier/meson,pexip/meson,fmuellner/meson,QuLogic/meson,wberrier/meson,centricular/meson,MathieuDuponchelle/meson,becm/meson,trhd/meson,ernestask/meson,wberrier/meson,wberrier/meson,mesonbuild/meson,becm/meson,jpakkane/meson,becm/meson,pexip/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,QuLogic/meson,thiblahute/meson,centricular/meson,ernestask/meson,ernestask/meson,trhd/meson,fmuellner/meson,aaronp24/meson,becm/meson,thiblahute/meson,QuLogic/meson,mesonbuild/meson,jpakkane/meson,becm/meson,trhd/meson,becm/meson,thiblahute/meson,rhd/meson,fmuellner/meson,jeandet/meson,thiblahute/meson,ernestask/meson,aaronp24/meson,pexip/meson,pexip/meson,jpakkane/meson,jeandet/meson,rhd/meson,jeandet/meson,mesonbuild/meson,becm/meson,centricular/meson,aaronp24/meson,jpakkane/meson,centricular/meson,fmuellner/meson,pexip/meson,trhd/meson,QuLogic/meson,mesonbuild/meson,fmuellner/meson,aaronp24/meson,centricular/meson,aaronp24/meson,mesonbuild/meson,pexip/meson,thiblahute/meson,mesonbuild/meson,rhd/meson,pexip/meson,rhd/meson,rhd/meson,becm/meson,MathieuDuponchelle/meson,becm/meson,wberrier/meson,ernestask/meson,jeandet/meson,trhd/meson,thiblahute/meson,jeandet/meson,fmuellner/meson,fmuellner/meson,trhd/meson,rhd/meson,thiblahute/meson,wberrier/meson,thiblahute/meson,jeandet/meson,jpakkane/meson,ernestask/meson,aaronp24/meson,trhd/meson,fmuellner/meson,QuLogic/meson,trhd/meson,aaronp24/meson,jeandet/meson,centricular/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,wberrier/meson,centricular/meson,MathieuDuponchelle/meson,pexip/meson,ernestask/meson,jeandet/meson,thiblahute/meson,centricular/meson,MathieuDuponchelle/meson,aaronp24/meson,ernestask/meson,rhd/meson,rhd/meson,MathieuDuponchelle/meson,mesonbuild/meson,trhd/meson,fmuellner/meson,jeandet/meson,jpakkane/meson,pexip/meson,MathieuDuponchelle/meson,QuLogic/meson,wberrier/meson,mesonbuild/meson,aaronp24/meson,becm/meson,mesonbuild/meson,MathieuDuponchelle/meson,QuLogic/meson,jpakkane/meson,ernestask/meson,jpakkane/meson