clarify on_missing_args help text

Author: chenyukang

compiler/rustc_passes/src/errors.rs

@@ -1359,9 +1359,7 @@ pub(crate) struct OnMoveMalformedFormatLiterals {
 
 #[derive(Diagnostic)]
 #[diag("unknown parameter `{$name}`")]
-#[help(
-    r#"the only supported format parameter for `#[diagnostic::on_missing_args]` is {"`{This}`"}"#
-)]
+#[help(r#"use {"`{This}`"} to refer to the macro name"#)]
 pub(crate) struct OnMissingArgsMalformedFormatLiterals {
     pub name: Symbol,
 }

tests/ui/diagnostic_namespace/on_missing_args/report_warning_on_invalid_formats.rs

@@ -2,7 +2,7 @@
 #![feature(diagnostic_on_missing_args)]
 
 #[diagnostic::on_missing_args(
-    message = "pair! is missing {T}",
+    message = "{T}! is missing arguments",
     //~^ WARN unknown parameter `T`
 )]
 macro_rules! pair {

tests/ui/diagnostic_namespace/on_missing_args/report_warning_on_invalid_formats.stderr

@@ -1,10 +1,10 @@
 warning: unknown parameter `T`
-  --> $DIR/report_warning_on_invalid_formats.rs:5:34
+  --> $DIR/report_warning_on_invalid_formats.rs:5:17
    |
-LL |     message = "pair! is missing {T}",
-   |                                  ^
+LL |     message = "{T}! is missing arguments",
+   |                 ^
    |
-   = help: the only supported format parameter for `#[diagnostic::on_missing_args]` is `{This}`
+   = help: use `{This}` to refer to the macro name
    = note: `#[warn(malformed_diagnostic_format_literals)]` (part of `#[warn(unknown_or_malformed_diagnostic_attributes)]`) on by default
 
 warning: 1 warning emitted