Document that user-defined functions and methods are unsupported
Document that user-defined functions and methods are unsupported

Fixes #1870
ernestask/meson,aaronp24/meson,wberrier/meson,becm/meson,aaronp24/meson,MathieuDuponchelle/meson,mesonbuild/meson,MathieuDuponchelle/meson,aaronp24/meson,thiblahute/meson,ernestask/meson,jeandet/meson,fmuellner/meson,mesonbuild/meson,MathieuDuponchelle/meson,jeandet/meson,jpakkane/meson,jeandet/meson,aaronp24/meson,becm/meson,QuLogic/meson,pexip/meson,ernestask/meson,QuLogic/meson,QuLogic/meson,wberrier/meson,pexip/meson,jpakkane/meson,becm/meson,becm/meson,mesonbuild/meson,QuLogic/meson,jeandet/meson,ernestask/meson,thiblahute/meson,thiblahute/meson,ernestask/meson,wberrier/meson,wberrier/meson,jeandet/meson,fmuellner/meson,thiblahute/meson,becm/meson,aaronp24/meson,fmuellner/meson,jpakkane/meson,fmuellner/meson,trhd/meson,mesonbuild/meson,ernestask/meson,thiblahute/meson,trhd/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,jeandet/meson,QuLogic/meson,trhd/meson,mesonbuild/meson,wberrier/meson,trhd/meson,jeandet/meson,QuLogic/meson,jeandet/meson,thiblahute/meson,thiblahute/meson,jpakkane/meson,ernestask/meson,pexip/meson,jeandet/meson,fmuellner/meson,trhd/meson,becm/meson,pexip/meson,trhd/meson,jpakkane/meson,QuLogic/meson,becm/meson,aaronp24/meson,thiblahute/meson,ernestask/meson,pexip/meson,mesonbuild/meson,wberrier/meson,MathieuDuponchelle/meson,trhd/meson,pexip/meson,aaronp24/meson,fmuellner/meson,fmuellner/meson,becm/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,jpakkane/meson,MathieuDuponchelle/meson,aaronp24/meson,thiblahute/meson,MathieuDuponchelle/meson,becm/meson,fmuellner/meson,wberrier/meson,QuLogic/meson,fmuellner/meson,aaronp24/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,wberrier/meson,trhd/meson,jpakkane/meson,pexip/meson,jpakkane/meson,ernestask/meson,QuLogic/meson,becm/meson,pexip/meson,pexip/meson,trhd/meson