Add note about interactions in operators (#18124)

* Add note about interactions in operators

Fixes #17988.  See #17988 (comment)

The null conditional operators and null forgiving operators interact in interesting ways. Add notes on both pages to explain those interactions.

* respond to feedback

* typos

* rework note based on feedback.

* one final set of changes

docs/csharp/language-reference/operators/member-access-operators.md

@@ -132,6 +132,9 @@ In the preceding example, if you don't use the `??` operator, `numbers?.Length <
 
 The null-conditional member access operator `?.` is also known as the Elvis operator.
 
+> [!NOTE]
+> In C# 8, the null-conditional operators interacts with the [null-forgiving operator](null-forgiving.md) in an unexpected way. For example, the expression `x?.y!.z` is parsed as `(x?.y)!.z`. Due to this interpretation, `z` is evaluated even if `x` is `null`, which may result in a <xref:System.NullReferenceException>.
+
 ### Thread-safe delegate invocation
 
 Use the `?.` operator to check if a delegate is non-null and invoke it in a thread-safe way (for example, when you [raise an event](../../../standard/events/how-to-raise-and-consume-events.md)), as the following code shows:

docs/csharp/language-reference/operators/null-forgiving.md

@@ -13,6 +13,9 @@ Available in C# 8.0 and later, the unary postfix `!` operator is the null-forgiv
 
 The null-forgiving operator has no effect at run time. It only affects the compiler's static flow analysis by changing the null state of the expression. At run time, expression `x!` evaluates to the result of the underlying expression `x`.
 
+> [!NOTE]
+> In C# 8 the null-forgiving operator interacts with the [null-conditional operators](member-access-operators.md#null-conditional-operators--and-) in an unexpected way. The expression `x?.y!.z` is parsed as `(x?.y)!.z`. Due to this interpretation `z` is evaluated even if `x` is `null`, which may result in a <xref:System.NullReferenceException>.
+
 For more information about the nullable reference types feature, see [Nullable reference types](../builtin-types/nullable-reference-types.md).
 
 ## Examples