message: Document "requireAccess".
diff: @@ -56,7 +56,17 @@ public final class SecretKeyAccess {
  return INSTANCE;
  }
  - /** Throws an exception if the passed in SecretKeyAccess is null, otherwise returns it. */
  + /**
  + * Throws an exception if the passed in {@link SecretKeyAccess} is null, otherwise returns it.
  + *
  + * <p>Note: Tink has two types of APIs, some which take a nullable {@code SecretKeyAccess}, and
  + * some which take a {@code SecretKeyAccess} without annotation. When an API takes a nullable
  + * {@code SecretKeyAccess}, this indicates that proper usage may call it with {@code null}, hence
  + * we typically want to throw a checked exception and {@code requireAccess} here is appropriate.
  + * Conversely, if an API takes an unannotated {@code SecretKeyAccess}, this indicates that the API
  + * always requires a non-null object. In this case, using it with null warrants should usually
  + * throw a null pointer exception (and one does not want to use {@code requireAccess}).
  + */
  @CanIgnoreReturnValue
  public static SecretKeyAccess requireAccess(@Nullable SecretKeyAccess access)
  throws GeneralSecurityException {
  