[CALCITE-7393] Support RelDataTypeDigest

fix missing function comment

Author: zhuwenzhuang

core/src/main/java/org/apache/calcite/rel/type/RelDataTypeImpl.java

@@ -334,7 +334,9 @@ protected abstract void generateTypeString(
       boolean withDetail);
 
   /**
-   * Init the lazy digest computing field.
+   * Init the lazy digest computing field {@link #innerDigest}.
+   * This should be called in every non-abstract subclass
+   * constructor once the type is fully defined.
    */
   @SuppressWarnings("method.invocation.invalid")
   protected void computeDigest(@UnknownInitialization RelDataTypeImpl this) {