Reword return type deprecation message

It is triggered for all classes (not only Symfony-ones), so we cannot be
precise on when it will be added. However, we must do a "call to action" to
prepare users for Symfony 6 (and potentially other package releases).

Author: wouterj

DebugClassLoader.php

@@ -560,7 +560,7 @@ public function checkAnnotations(\ReflectionClass $refl, string $class): array
                     if ('docblock' === $this->patchTypes['force']) {
                         $this->patchMethod($method, $returnType, $declaringFile, $normalizedType);
                     } elseif ('' !== $declaringClass && $this->patchTypes['deprecations']) {
-                        $deprecations[] = sprintf('Method "%s::%s()" will return "%s" as of its next major version. Doing the same in %s "%s" will be required when upgrading.', $declaringClass, $method->name, $normalizedType, interface_exists($declaringClass) ? 'implementation' : 'child class', $className);
+                        $deprecations[] = sprintf('Method "%s::%s()" might add "%s" as a native return type declaration in the future. Do the same in %s "%s" now to avoid errors or add an explicit @return annotation to suppress this message.', $declaringClass, $method->name, $normalizedType, interface_exists($declaringClass) ? 'implementation' : 'child class', $className);
                     }
                 }
             }