docs: clarify NaN, inf, -inf
docs: clarify NaN, inf, -inf

closes #2089

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