Use 1-ZST terminology

Author: gnzlbg

reference/src/layout/unions.md

@@ -34,10 +34,11 @@ the `union`, etc.
 
 ### Unions with default layout ("`repr(Rust)`")
 
-The default layout of Rust unions is **unspecified**. 
+Except for the guarantees provided below for some specific cases, the default
+layout of Rust unions is, _in general_, **unspecified**.
 
-That is, there are no guarantees about the offset of the fields, whether all
-fields have the same offset, etc.
+That is, there are no _general_ guarantees about the offset of the fields,
+whether all fields have the same offset, etc.
 
 <details><summary><b>Rationale</b></summary>
 
@@ -59,8 +60,8 @@ you have to use `#[repr(C)]`.
 The layout of unions with a single non-zero-sized field is the same as the
 layout of that field if:
 
-* that field has no padding bits, and
-* the alignment requirement of all zero-sized fields is 1.
+* all zero-sized fields are [1-ZST], and
+* the non-zero sized field has no padding bits.
 
 For example, here:
 
@@ -83,7 +84,7 @@ union U0 {
 
 the union `U0` has the same layout as `SomeStruct`, because `SomeStruct` has no
 padding bits - it is equivalent to an `i32` due to `repr(transparent)` - and
-because the alignment of `Zst` is 1.
+because `Zst` is a [1-ZST].
 
 On the other hand, here:
 
@@ -104,9 +105,10 @@ assert_eq!(align_of::<Zst2>(), 16);
 # }
 ```
 
-the alignment requirement of `Zst2` is not 1, and `SomeOtherStruct` has an
-unspecified layout and could contain padding bits. Therefore, the layout of `U1`
-is **unspecified**.
+the layout of `U1` is **unspecified** because:
+
+* `Zst2` is not a [1-ZST], and
+* `SomeOtherStruct` has an unspecified layout and could contain padding bits.
 
 ### C-compatible layout ("repr C")
 
@@ -117,3 +119,5 @@ a field type to obtain a pointer to any field, and vice versa.
 
 [6.5.8.5]: http://port70.net/~nsz/c/c11/n1570.html#6.5.8p5
 [6.7.2.1.16]: http://port70.net/~nsz/c/c11/n1570.html#6.7.2.1p16
+
+[1-ZST]: ../glossary.md#zero-sized-type--zst