Add forward reference for non_exhaustive exception

The normative rule about destructuring and capturing does not itself
state the exception for `non_exhaustive` enums.  That exception
follows below.  Let's add an admonition to link down to it so it's not
missed.

It's not great having this duplication; it'd be better if we weren't
restating this fact twice.  But it's not clear what to do about this.
The first statement is making a broader statement about tuples,
structs, and single-variant enums.  The section that follows below is
focused on discriminants.  We'll think about this more later.

Author: traviscross

src/types/closure.md

@@ -205,6 +205,9 @@ c();
 r[type.closure.capture.precision.wildcard.destructuring]
 Destructuring tuples, structs, and single-variant enums does not, by itself, cause a read or the place to be captured.
 
+> [!NOTE]
+> Enums marked with [`#[non_exhaustive]`][attributes.type-system.non_exhaustive] from other crates are always treated as having multiple variants. See *[type.closure.capture.precision.discriminants.non_exhaustive]*.
+
 ```rust,no_run
 struct S; // A non-`Copy` type.