Add diagnostic docs for extension types

I'm fairly sure there will be other diagnostics added later, so this
isn't everything, but it covers the diagnostics that are currently
implemented in the analyzer.

Change-Id: I27100c9a76e6db4fb2b0bf8b9bc6b0b2c326707f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/322442
Reviewed-by: Marya Belanger <mbelanger@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/src/error/codes.g.dart

@@ -258,7 +258,9 @@ class CompileTimeErrorCode extends AnalyzerErrorCode {
     'AWAIT_OF_EXTENSION_TYPE_NOT_FUTURE',
     "The 'await' expression can't be used for an expression with an extension "
         "type that is not a subtype of 'Future'.",
-    correctionMessage: "Try updating the extension type to implement 'Future'.",
+    correctionMessage:
+        "Try removing the `await`, or updating the extension type to implement "
+        "'Future'.",
   );
 
   ///  Parameters:
@@ -1672,8 +1674,8 @@ class CompileTimeErrorCode extends AnalyzerErrorCode {
   static const CompileTimeErrorCode
       EXTENSION_TYPE_CONSTRUCTOR_WITH_SUPER_INVOCATION = CompileTimeErrorCode(
     'EXTENSION_TYPE_CONSTRUCTOR_WITH_SUPER_INVOCATION',
-    "Extension type constructors can't include superinitializers.",
-    correctionMessage: "Try removing the superconstructor invocation.",
+    "Extension type constructors can't include super initializers.",
+    correctionMessage: "Try removing the super constructor invocation.",
   );
 
   ///  No parameters.