Improve javadoc for ReadableFieldPointer. (#16043)

Since #15175, the javadoc for ReadableFieldPointer is somewhat out of date. It says that
the pointer only points to the beginning of the field, but this is no longer true. This
patch updates the javadoc to be more accurate.

processing/src/main/java/org/apache/druid/frame/field/ReadableFieldPointer.java

@@ -20,10 +20,8 @@
 package org.apache.druid.frame.field;
 
 /**
- * Pointer to a field position in some memory. Only points to the beginning of the field, since all fields
- * can be read without knowing their entire length.
- *
- * See {@link org.apache.druid.frame.write.RowBasedFrameWriter} for details about the format.
+ * Pointer to a field position in some memory. See {@link org.apache.druid.frame.write.RowBasedFrameWriter} for details
+ * about the format.
  */
 public interface ReadableFieldPointer
 {
@@ -33,7 +31,8 @@ public interface ReadableFieldPointer
   long position();
 
   /**
-   * Length of the field.
+   * Length of the field. Never necessary to read a field, since all fields can be read without knowing their
+   * entire length. Provided because it may be useful for reading in a more optimal manner.
    */
   long length();
 }