add some documentation about the new 'no limit' option

runtime/src/main/java/org/capnproto/Arena.java

@@ -23,5 +23,5 @@ package org.capnproto;
 
 public interface Arena {
     public SegmentReader tryGetSegment(int id);
-    public void checkReadLimit(int numBytes);
+    public void checkReadLimit(int numWords);
 }

runtime/src/main/java/org/capnproto/ReaderArena.java

@@ -26,8 +26,8 @@ import java.nio.ByteBuffer;
 
 public final class ReaderArena implements Arena {
 
+    // Current limit. -1 means no limit.
     public long limit;
-    // current limit
 
     public final ArrayList<SegmentReader> segments;
 
@@ -47,6 +47,7 @@ public final class ReaderArena implements Arena {
     @Override
     public final void checkReadLimit(int numWords) {
         if (limit == -1) {
+            // No limit.
             return;
         } else if (numWords > limit) {
             throw new DecodeException("Read limit exceeded.");

runtime/src/main/java/org/capnproto/ReaderOptions.java

@@ -22,7 +22,18 @@
 package org.capnproto;
 
 public final class ReaderOptions {
+    /**
+       How many words are allowed to be read before an exception is thrown,
+       to protect against denial of service attacks.
+
+       -1 means "no limit".
+    */
     public final long traversalLimitInWords;
+
+    /**
+       How many pointer indirections deep a message may be before an exception
+       is thrown.
+    */
     public final int nestingLimit;
 
     public ReaderOptions(long traversalLimitInWords, int nestingLimit) {