Added a comment describing the need to use "!" operator

src/Polly.Core/ResilienceProperties.cs

@@ -29,6 +29,12 @@ public bool TryGetValue<TValue>(ResiliencePropertyKey<TValue> key, [MaybeNullWhe
             }
             else if (val is null)
             {
+                // We have to use null-forgiving operator "!" here to suppress a null-state analysis warning.
+                // The reason is the following. The output type "TValue" doesn't have any type constraints as
+                // "notnull", "class" or "struct", therefore the analyzer considers "TValue" as non-nullable
+                // and warns us that we're assigning "null" to it. But that's not correct, because "TValue"
+                // could be a nullable type, e.g. "string?", and assigning "null" to it is correct. Therefore
+                // it is reasonable to use "!" here to suppress the warning.
                 value = default!;
                 return true;
             }