Auto merge of #87836 - tsoutsman:master, r=petrochenkov

Change proc_macro::Diagnostics docs

With Rust 1.54 attributes can invoke function-like macros. This allows functions generated by macros to have non-generic documentation. This pull request changes the documentation of `proc_macro::Diagnostics` to be more specific and include a link to the level.
## Example
Before:
```markdown
Adds a new child diagnostic message to `self` with the level
identified by this method’s name with the given `message`.
```
After:
```markdown
Adds a new child diagnostic message to self with the [`Level::Error`]
level, and the given `message`.
```

library/proc_macro/src/diagnostic.rs

@@ -56,10 +56,9 @@ pub struct Diagnostic {
 
 macro_rules! diagnostic_child_methods {
     ($spanned:ident, $regular:ident, $level:expr) => {
-        /// Adds a new child diagnostic message to `self` with the level
-        /// identified by this method's name with the given `spans` and
-        /// `message`.
         #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
+        #[doc = concat!("Adds a new child diagnostics message to `self` with the [`",
+                        stringify!($level), "`] level, and the given `spans` and `message`.")]
         pub fn $spanned<S, T>(mut self, spans: S, message: T) -> Diagnostic
         where
             S: MultiSpan,
@@ -69,9 +68,9 @@ macro_rules! diagnostic_child_methods {
             self
         }
 
-        /// Adds a new child diagnostic message to `self` with the level
-        /// identified by this method's name with the given `message`.
         #[unstable(feature = "proc_macro_diagnostic", issue = "54140")]
+        #[doc = concat!("Adds a new child diagnostic message to `self` with the [`",
+                        stringify!($level), "`] level, and the given `message`.")]
         pub fn $regular<T: Into<String>>(mut self, message: T) -> Diagnostic {
             self.children.push(Diagnostic::new($level, message));
             self