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.

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

Author: perillo

doc/langref.html.in

@@ -6627,7 +6627,7 @@ 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>
       <p>
       These types can only ever have one possible value, and thus
@@ -6650,6 +6650,9 @@ 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>