SPARK-4457. Document how to build for Hadoop versions greater than 2.4

Author: Sandy Ryza <sandy@cloudera.com>

Closes #3322 from sryza/sandy-spark-4457 and squashes the following commits:

5e72b77 [Sandy Ryza] Feedback
0cf05c1 [Sandy Ryza] Caveat
be8084b [Sandy Ryza] SPARK-4457. Document how to build for Hadoop versions greater than 2.4

docs/building-spark.md

@@ -92,8 +92,11 @@ mvn -Pyarn -Phadoop-2.2 -Dhadoop.version=2.2.0 -DskipTests clean package
 # Apache Hadoop 2.3.X
 mvn -Pyarn -Phadoop-2.3 -Dhadoop.version=2.3.0 -DskipTests clean package
 
-# Apache Hadoop 2.4.X
-mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=2.4.0 -DskipTests clean package
+# Apache Hadoop 2.4.X or 2.5.X
+mvn -Pyarn -Phadoop-2.4 -Dhadoop.version=VERSION -DskipTests clean package
+
+Versions of Hadoop after 2.5.X may or may not work with the -Phadoop-2.4 profile (they were
+released after this version of Spark).
 
 # Different versions of HDFS and YARN.
 mvn -Pyarn-alpha -Phadoop-2.3 -Dhadoop.version=2.3.0 -Dyarn.version=0.23.7 -DskipTests clean package