Add limitation description for enum_variant_names

`enum_variant_names` will consider characters with no case to be a part
of prefixes/suffixes substring that are compared. This means `Foo1` and
`Foo2` has different prefixes (`Foo1` and `Foo2` prefix respeectively).
This applies to all non-ascii characters with no casing.

clippy_lints/src/enum_variants.rs

@@ -18,6 +18,12 @@ declare_clippy_lint! {
     /// Enumeration variant names should specify their variant,
     /// not repeat the enumeration name.
     ///
+    /// ### Limitations
+    /// Characters with no casing will be considered when comparing prefixes/suffixes
+    /// This applies to numbers and non-ascii characters without casing
+    /// e.g. `Foo1` and `Foo2` is considered to have different prefixes
+    /// (the prefixes are `Foo1` and `Foo2` respectively), as also `Bar螃`, `Bar蟹`
+    ///
     /// ### Example
     /// ```rust
     /// enum Cake {