Escape characters in doc comments in macros correctly

Previously they were escaped twice, both by `.escape_default()` and the debug view of strings (`{:?}`). This leads to things like newlines or tabs in documentation comments being `\\n`, but we unescape literals only once, ending up with `\n`.

This was hard to spot because CMark unescaped them (at least for `'` and `"`), but it did not do so in code blocks.

This also was the root cause of #7781. This issue was solved by using `.escape_debug()` instead of `.escape_default()`, but the real issue remained.
We can bring the `.escape_default()` back by now, however I didn't do it because it is probably slower than `.escape_debug()` (more work to do), and also in order to change the code the least.

crates/mbe/src/syntax_bridge.rs

@@ -213,7 +213,7 @@ fn doc_comment_text(comment: &ast::Comment) -> SmolStr {
 
     // Quote the string
     // Note that `tt::Literal` expect an escaped string
-    let text = format!("{:?}", text.escape_debug().to_string());
+    let text = format!("\"{}\"", text.escape_debug());
     text.into()
 }