Document the purpose of containsNullnessAnnotation() (#7578)

Author: mernst

checker/src/main/java/org/checkerframework/checker/nullness/NullnessAnnotatedTypeFactory.java

@@ -851,6 +851,9 @@ protected AnnotationMirror greatestLowerBoundWithElements(
    * <p>This method ignores aliases of nullness annotations that are declaration annotations,
    * because they may apply to inner types.
    *
+   * <p>This method is used only for issuing an error when a nullness annotation is written in a
+   * place it should not be.
+   *
    * @param annoTrees a list of annotations that the Java parser attached to the variable/method
    *     declaration; null if this type is not from such a location. This is a list of extra
    *     annotations to check, in addition to those on the type.
@@ -871,8 +874,9 @@ protected boolean containsNullnessAnnotation(
    * <p>This method ignores aliases of nullness annotations that are declaration annotations,
    * because they may apply to inner types.
    *
-   * <p>Clients that are processing a field or variable definition, or a method return type, should
-   * call {@link #containsNullnessAnnotation(List, Tree)} instead.
+   * <p>This method is used only for issuing an error when a nullness annotation is written in a
+   * place it should not be. Clients that are processing a field or variable definition, or a method
+   * return type, should call {@link #containsNullnessAnnotation(List, Tree)} instead.
    *
    * @param annoTrees a list of annotations to check
    * @return true if some annotation is a nullness annotation