Add docs for new @mustBeConst diagnostic (#5689)

Author: parlough

src/content/tools/diagnostic-messages.md

@@ -15089,6 +15089,40 @@ class C {
 }
 ```
 
+### non_const_argument_for_const_parameter
+
+_Argument '{0}' must be a constant._
+
+#### Description
+
+The analyzer produces this diagnostic when a parameter is
+annotated with the `@mustBeConst` annotation and the
+corresponding argument is not a constant expression.
+
+#### Example
+
+The following code produces this diagnostic on the invocation of
+the function `f` because the value of the argument passed to the
+function `g` isn't a constant:
+
+```dart
+import 'package:meta/meta.dart' show mustBeConst;
+int f(int value) => g([!value!]);
+int g(@mustBeConst int value) => value + 1;
+```
+
+#### Common fixes
+
+If a suitable constant is available to use, then replace the argument
+with a constant:
+
+```dart
+import 'package:meta/meta.dart' show mustBeConst;
+const v = 3;
+int f() => g(v);
+int g(@mustBeConst int value) => value + 1;
+```
+
 ### non_const_call_to_literal_constructor
 
 _This instance creation must be 'const', because the {0} constructor is marked