docs: Add mention of target made by gnome.gtkdoc()
docs: Add mention of target made by gnome.gtkdoc()

Mentioned in #1844
rhd/meson,ernestask/meson,trhd/meson,pexip/meson,becm/meson,jeandet/meson,pexip/meson,becm/meson,aaronp24/meson,ernestask/meson,jpakkane/meson,becm/meson,mesonbuild/meson,thiblahute/meson,becm/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,jpakkane/meson,pexip/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,fmuellner/meson,aaronp24/meson,wberrier/meson,jpakkane/meson,fmuellner/meson,rhd/meson,mesonbuild/meson,thiblahute/meson,trhd/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,QuLogic/meson,thiblahute/meson,jpakkane/meson,trhd/meson,wberrier/meson,aaronp24/meson,QuLogic/meson,thiblahute/meson,QuLogic/meson,rhd/meson,becm/meson,thiblahute/meson,fmuellner/meson,jpakkane/meson,thiblahute/meson,QuLogic/meson,ernestask/meson,rhd/meson,jpakkane/meson,wberrier/meson,mesonbuild/meson,aaronp24/meson,rhd/meson,jpakkane/meson,ernestask/meson,QuLogic/meson,mesonbuild/meson,fmuellner/meson,wberrier/meson,pexip/meson,pexip/meson,mesonbuild/meson,ernestask/meson,becm/meson,trhd/meson,wberrier/meson,aaronp24/meson,pexip/meson,jeandet/meson,wberrier/meson,becm/meson,MathieuDuponchelle/meson,QuLogic/meson,thiblahute/meson,fmuellner/meson,trhd/meson,becm/meson,jeandet/meson,jeandet/meson,rhd/meson,rhd/meson,MathieuDuponchelle/meson,wberrier/meson,becm/meson,fmuellner/meson,ernestask/meson,trhd/meson,thiblahute/meson,wberrier/meson,mesonbuild/meson,fmuellner/meson,QuLogic/meson,ernestask/meson,rhd/meson,pexip/meson,MathieuDuponchelle/meson,trhd/meson,aaronp24/meson,jpakkane/meson,jeandet/meson,ernestask/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,aaronp24/meson,thiblahute/meson,aaronp24/meson,mesonbuild/meson,fmuellner/meson,QuLogic/meson,QuLogic/meson,trhd/meson,aaronp24/meson,trhd/meson,pexip/meson,jeandet/meson,mesonbuild/meson,jeandet/meson,pexip/meson,fmuellner/meson,ernestask/meson,jeandet/meson