mention how unions interact with dropping

Author: RalfJung

src/items/unions.md

@@ -79,6 +79,16 @@ u.f1 = 2;
 Commonly, code using unions will provide safe wrappers around unsafe union
 field accesses.
 
+## Unions and `Drop`
+
+When a union is dropped, it cannot know which of its fields needs to be dropped.
+For this reason, all union fields must either be of a `Copy` type or of the
+shape `ManuallyDrop<_>`.  This ensures that a union does not need to drop
+anything when it goes out of scope.
+
+Like for structs and enums, it is possible to `impl Drop` for a union to
+manually define what happens when it gets dropped.
+
 ## Pattern matching on unions
 
 Another way to access union fields is to use pattern matching. Pattern matching

src/types/union.md

@@ -6,7 +6,7 @@ a [`union` item][item].
 Unions have no notion of an "active field". Instead, every union access
 transmutes parts of the content of the union to the type of the accessed
 field. Since transmutes can cause unexpected or undefined behaviour, `unsafe` is
-required to read from a union field or to write to a field that doesn't
+required to read from a union field, or to write to a field that doesn't
 implement [`Copy`]. See the [item] documentation for further details.
 
 The memory layout of a `union` is undefined by default, but the `#[repr(...)]`