Add a warning to Delimiter::None that rustc currently does not respect it.

It does not provide the behaviour it is indicated to provide when used in
a proc_macro context.

Author: CensoredUsername

src/lib.rs

@@ -685,6 +685,16 @@ pub enum Delimiter {
     /// operator priorities in cases like `$var * 3` where `$var` is `1 + 2`.
     /// Invisible delimiters may not survive roundtrip of a token stream through
     /// a string.
+    ///
+    /// <div class="warning">
+    ///
+    /// Note: rustc currently (1.77 and below) ignores the grouping of tokens
+    /// delimited by `None` in the output of a proc_macro. Thus it will not
+    /// preserve operator priorities as indicated above. The other `Delimiter`
+    /// variants should be used instead in this context. For details, see
+    /// [rust-lang/rust#67062](https://github.com/rust-lang/rust/issues/67062).
+    ///
+    /// </div>
     None,
 }