add docs for zero bit types and pointers to zero bit types
add docs for zero bit types and pointers to zero bit types

closes #1561

zig-lang/zig,raulgrell/zig,andrewrk/zig,raulgrell/zig,andrewrk/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig