Update documentation.

This commit updates the unstable book and diagnostics to reflect that
the `#[non_exhaustive]` attribute is now available for enum variants.

src/doc/unstable-book/src/language-features/non-exhaustive.md

@@ -7,10 +7,12 @@ The tracking issue for this feature is: [#44109]
 ------------------------
 
 The `non_exhaustive` gate allows you to use the `#[non_exhaustive]` attribute
-on structs and enums. When applied within a crate, users of the crate will need
-to use the `_` pattern when matching enums and use the `..` pattern when
-matching structs. Structs marked as `non_exhaustive` will not be able to be
-created normally outside of the defining crate. This is demonstrated below:
+on structs, enums and enum variants. When applied within a crate, users of the
+crate will need to use the `_` pattern when matching enums and use the `..`
+pattern when matching structs. Enum variants cannot be matched against.
+Structs and enum variants marked as `non_exhaustive` will not be able to
+be created normally outside of the defining crate. This is demonstrated
+below:
 
 ```rust,ignore (pseudo-Rust)
 use std::error::Error as StdError;
@@ -72,4 +74,3 @@ let config = Config { window_width: 640, window_height: 480 };
 // when marked non_exhaustive.
 let &Config { window_width, window_height, .. } = config;
 ```
-

src/librustc_typeck/diagnostics.rs

@@ -4341,11 +4341,12 @@ foo.method(); // Ok!
 "##,
 
 E0638: r##"
-This error indicates that the struct or enum must be matched non-exhaustively
-as it has been marked as `non_exhaustive`.
+This error indicates that the struct, enum or enum variant must be matched
+non-exhaustively as it has been marked as `non_exhaustive`.
 
 When applied within a crate, downstream users of the crate will need to use the
 `_` pattern when matching enums and use the `..` pattern when matching structs.
+Downstream crates cannot match against non-exhaustive enum variants.
 
 For example, in the below example, since the enum is marked as
 `non_exhaustive`, it is required that downstream crates match non-exhaustively
@@ -4390,10 +4391,10 @@ Similarly, for structs, match with `..` to avoid this error.
 "##,
 
 E0639: r##"
-This error indicates that the struct or enum cannot be instantiated from
-outside of the defining crate as it has been marked as `non_exhaustive` and as
-such more fields/variants may be added in future that could cause adverse side
-effects for this code.
+This error indicates that the struct, enum or enum variant cannot be
+instantiated from outside of the defining crate as it has been marked
+as `non_exhaustive` and as such more fields/variants may be added in
+future that could cause adverse side effects for this code.
 
 It is recommended that you look for a `new` function or equivalent in the
 crate's documentation.