langref: improve zero bit types documentation

In the Zero Bit Types section, clarify that a non empty zero bit union
just needs to have all fields of zero bit type, instead of only one
field of zero bit type.

Add a doctest.

Clarify that empty struct, enum, union and error set are currently
problematic.

Author: perillo

doc/langref.html.in

@@ -6627,8 +6627,36 @@ test "peer type resolution: *const T and ?*T" {
           <li>{#link|Arrays#} and {#link|Vectors#} with len 0, or with an element type that is a zero bit type.</li>
           <li>An {#link|enum#} with only 1 tag.</li>
           <li>A {#link|struct#} with all fields being zero bit types.</li>
-          <li>A {#link|union#} with only 1 field which is a zero bit type.</li>
+          <li>An {#link|union#} with all fields being zero bit types.</li>
       </ul>
+      {#code_begin|test|test_zero_bit_types#}
+const std = @import("std");
+const testing = std.testing;
+
+test "zero-bit primitive types" {
+    inline for (.{
+        void,
+        u0,
+        i0,
+    }) |typ| {
+        try testing.expect(@sizeOf(typ) == 0);
+    }
+}
+
+test "zero-bit types" {
+    inline for (.{
+        [0]u32,
+        [10]u0,
+        @Vector(0, u32),
+        @Vector(10, u0),
+        enum { empty },
+        struct { a: u0, b: void },
+        union { a: u0, b: void },
+    }) |typ| {
+        try testing.expect(@sizeOf(typ) == 0);
+    }
+}
+      {#code_end#}
       <p>
       These types can only ever have one possible value, and thus
       require 0 bits to represent. Code that makes use of these types is
@@ -6650,6 +6678,9 @@ export fn entry() void {
   15:	c3                   	retq   </code></pre>
       <p>These assembly instructions do not have any code associated with the void values -
       they only perform the function call prologue and epilogue.</p>
+      <p>Using empty {#link|struct#}, empty {#link|enum#}, empty {#link|union#},
+      and empty {#link|Error Set Type#} is problematic, and may be a compile error in the future.
+      For details on this subscribe to <a href="https://github.com/ziglang/zig/issues/13807">this issue</a>.
 
       {#header_open|void#}
       <p>