SPARK-2148 Add link to requirements for custom equals() and hashcode() methods

https://issues.apache.org/jira/browse/SPARK-2148

Author: Andrew Ash <andrew@andrewash.com>

Closes #1092 from ash211/SPARK-2148 and squashes the following commits:

93513df [Andrew Ash] SPARK-2148 Add link to requirements for custom equals() and hashcode() methods

docs/programming-guide.md

@@ -762,6 +762,11 @@ val counts = pairs.reduceByKey((a, b) => a + b)
 We could also use `counts.sortByKey()`, for example, to sort the pairs alphabetically, and finally
 `counts.collect()` to bring them back to the driver program as an array of objects.
 
+**Note:** when using custom objects as the key in key-value pair operations, you must be sure that a
+custom `equals()` method is accompanied with a matching `hashCode()` method.  For full details, see
+the contract outlined in the [Object.hashCode()
+documentation](http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()).
+
 </div>
 
 <div data-lang="java" markdown="1">
@@ -794,6 +799,10 @@ JavaPairRDD<String, Integer> counts = pairs.reduceByKey((a, b) -> a + b);
 We could also use `counts.sortByKey()`, for example, to sort the pairs alphabetically, and finally
 `counts.collect()` to bring them back to the driver program as an array of objects.
 
+**Note:** when using custom objects as the key in key-value pair operations, you must be sure that a
+custom `equals()` method is accompanied with a matching `hashCode()` method.  For full details, see
+the contract outlined in the [Object.hashCode()
+documentation](http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html#hashCode()).
 
 </div>