Update ref-returns.md (dotnet#2914)

* Update ref-returns.md

It seems like there's a missing 'not' when explaining why ref return values cannot be part of `async` methods. I might be wrong but it would make sense that since their values are not know, they cannot be in `async` methods.

* Update ref-returns.md

Rewrote the sentence to avoid any confusion concerning `async` methods.

Author: Francois Joly

Diff for: docs/csharp/programming-guide/classes-and-structs/ref-returns.md

@@ -38,7 +38,7 @@ There are some restrictions on the value that a method can return as a reference
 
 - The return value cannot be a constant, an enumeration member, or a property of a `class` or `struct`. Attempting to return these generates compiler error CS8156, "An expression cannot be used in this context because it may not be returned by reference."
 
-In addition, because an asynchronous method may return before it has finished execution and its return value is known, reference return values are not allowed on `async` methods.
+In addition, because an asynchronous method may return before it has finished execution, while its return value is still unknown, reference return values are not allowed on async methods.
 
 ## Defining a ref return value