tweak MaybeNull attribute examples (#1456)

Author: RexJaeschke

standard/attributes.md

@@ -982,23 +982,14 @@ Specifies that a non-nullable return value may be null.
 
 > *Example*: Consider the following generic method:
 >
-> <!-- TODO:  This sample no longer generates the error in C# 9. This could be part of the new nullable constraints. Was: , expectedErrors:["CS8627"]-->
-> <!-- Example: {template:"code-in-class-lib", name:"MaybeNull1Attribute", replaceEllipsis:true, customEllipsisReplacements: ["return default;"]} -->
-> ```csharp
-> #nullable enable
-> public T? Find<T>(IEnumerable<T> sequence, Func<T, bool> predicate) { ... }
-> ```
->
-> The idea of this code is that if `T` is replaced by `string`, `T?` becomes a nullable annotation. However, this code is not legal because `T` is not constrained to be a reference type. However, adding this attribute solves the problem:
->
 > <!-- Example: {template:"code-in-class-lib", name:"MaybeNull2Attribute", replaceEllipsis:true, customEllipsisReplacements: ["return default;"]} -->
 > ```csharp
 > #nullable enable
 > [return: MaybeNull]
 > public T Find<T>(IEnumerable<T> sequence, Func<T, bool> predicate) { ... }
 > ```
 >
-> The attribute informs callers that the contract implies a non-nullable type, but the return value may actually be `null`. *end example*
+> Without the attribute the compiler might generate a warning if the method could return `null`. The presence of the attribute suppresses that warning. *end example*
 
 #### 23.5.7.7 The MaybeNullWhen attribute