Rollup merge of rust-lang#96682 - nnethercote:show-invisible-delims, r=petrochenkov

Show invisible delimeters (within comments) when pretty printing.

Because invisible syntax is really hard to work with!

r? `@petrochenkov`

Author: matthiaskrgr

proc_macro/src/lib.rs

@@ -703,11 +703,12 @@ pub enum Delimiter {
     /// `[ ... ]`
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     Bracket,
-    /// `Ø ... Ø`
+    /// `/*«*/ ... /*»*/`
     /// An invisible delimiter, that may, for example, appear around tokens coming from a
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.
-    /// Invisible delimiters might not survive roundtrip of a token stream through a string.
+    /// Invisible delimiters are not directly writable in normal Rust code except as comments.
+    /// Therefore, they might not survive a roundtrip of a token stream through a string.
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     None,
 }