Elaborate on how union constructors produce union values

Author: chorman0773

Diff for: src/types/union.md

@@ -25,10 +25,16 @@ The memory layout of a `union` is undefined by default (in particular, fields do
 fix a layout.
 
 r[type.union.value]
-A value of a union type consists of a sequence of bytes, corresponding to each [value byte][type.struct.value.value-bytes]. The value bytes of a union are represented exactly. Each [padding byte][type.struct.value.padding] is set to uninit.
+A value of a union type consists of a sequence of bytes, corresponding to each [value byte][type.struct.value.value-bytes]. The value bytes of a union are represented exactly. Each [padding byte][type.struct.value.padding] is set to uninit when encoded.
 
 > [!NOTE]
-> When a union value is constructed or a field is read/written to, the value of that field is encoded or decoded appropriately.
+> A given value byte is guaranteed allowed to be uninit if it is padding in any field, recursively expanding union fields. Whether a byte of a union is allowed to be uninit in any other case is not yet decided.
+
+r[type.union.constructor]
+The constructor of a union type encodes the initialized field value into the corresponding bytes of the union, and sets all bytes that are not used by the field to uninit.
+
+r[type.union.field-access]
+When a field is written to by a field access expression, the value written is encoded into the corresponding bytes of the union. When a field is read, the value of that field is decoded from the corresponding bytes.
 
 [`Copy`]: ../special-types-and-traits.md#copy
 [item]: ../items/unions.md