Document fields of Link/Node

Summary:
Clarify fields in each data type, in particular the different function of
timestamp in the two data types.

Test Plan: Just added comments.  It still compiles

Reviewers: dhruba, emayanke

Reviewed By: emayanke

Differential Revision: https://reviews.facebook.net/D9957

src/java/com/facebook/LinkBench/Link.java

@@ -72,12 +72,26 @@ public Link clone() {
     return l;
   }
 
+  /** The node id of the source of directed edge */
   public long id1;
-  public long link_type;
+
+  /** The node id of the target of directed edge */
   public long id2;
+
+  /** Type of link */
+  public long link_type;
+
+  /** Visibility mode */
   public byte visibility;
-  public byte[] data;
+
+  /** Version of link */
   public int version;
+
+  /** time is the sort key for links.  Often it contains a timestamp,
+      but it can be used as a arbitrary user-defined sort key. */
   public long time;
 
+  /** Arbitrary payload data */
+  public byte[] data;
+
 }

src/java/com/facebook/LinkBench/Node.java

@@ -22,10 +22,19 @@
  * @author tarmstrong
  */
 public class Node {
+  /** Unique identifier for node */
   public long id;
+
+  /** Type of node */
   public int type;
+
+  /** Version of node: typically updated on every change */
   public long version;
+
+  /** Last update time of node as UNIX timestamp */
   public int time;
+
+  /** Arbitrary payload data */
   public byte data[];
 
   public Node(long id, int type, long version, int time,