Copy docs to getter in Properties macro #4184

Job	Run time
build GTK binaries with gvsbuild	13s
build gtk-rs on Windows (cairo, true, --features png,pdf,svg,ps,use_glib,v1_16,freetype,script,wi...	2m 56s
build gtk-rs on Windows (gdk-pixbuf, true, --features v2_42)	2m 46s
build gtk-rs on Windows (gio, true, --features v2_80)	3m 37s
build gtk-rs on Windows (glib, true, --features v2_80)	4m 16s
build gtk-rs on Windows (glib-build-tools, false)	2m 48s
build gtk-rs on Windows (graphene, false)	2m 7s
build gtk-rs on Windows (pango, true, --features v1_52)	2m 4s
build gtk-rs on Windows (pangocairo, true)	2m 45s
build gtk-rs on Windows (examples, false, --bins --examples --all-features)	3m 26s
	26m 58s

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