Add comment to BsonBinary#getData indicating that it returns the internal copy of the array

JAVA-3634

Author: jyemin

bson/src/main/org/bson/BsonBinary.java

@@ -165,6 +165,9 @@ public byte getType() {
     /**
      * Gets the data of this Binary.
      *
+     * This method returns the internal copy of the byte array, so only modify the contents of the returned array if the intention is to
+     * change the state of this instance.
+     *
      * @return the data
      */
     public byte[] getData() {