[SPARK-9521] [DOCS] Addendum. Require Maven 3.3.3+ in the build

Follow on for #7852: Building Spark doc needs to refer to new Maven requirement too

Author: Sean Owen <sowen@cloudera.com>

Closes #7905 from srowen/SPARK-9521.2 and squashes the following commits:

73285df [Sean Owen] Follow on for #7852: Building Spark doc needs to refer to new Maven requirement too

docs/building-spark.md

@@ -7,7 +7,8 @@ redirect_from: "building-with-maven.html"
 * This will become a table of contents (this text will be scraped).
 {:toc}
 
-Building Spark using Maven requires Maven 3.0.4 or newer and Java 7+.
+Building Spark using Maven requires Maven 3.3.3 or newer and Java 7+.
+The Spark build can supply a suitable Maven binary; see below.
 
 # Building with `build/mvn`