This commit removes the check that disallows the `#[non_exhaustive]`
attribute from being placed on enum variants and removes the associated
tests.

Further, this commit lowers the visibility of enum variant constructors
when the variant is marked as non-exhaustive.

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

This commit adds support for non-exhaustive enum variants in rustdoc,
extending the existing support for non-exhaustive enums and structs.