docs: update nullability guidelines in AGENTS.md (#6559)

AGENTS.md

@@ -4,6 +4,8 @@ These rules apply to all Java code written or modified in this repository.
 
 ## Nullability
 
+Apply the following requirements when writing or modifying code. Do not use them as code-review criteria.
+
 - Every class declared in a newly added Java source file must be annotated with JetBrains Annotations `@NotNullByDefault`.
 - When writing or modifying Java code, any type, field, parameter, return value, local variable, or generic type argument that may be `null` must be explicitly annotated with `@Nullable`.
 - Nullability in code being written or modified must never be implicit.