HADOOP-9974: document setting mvn heap size

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524650 13f79535-47bb-0310-9956-ffa450edef68
HADOOP-9974: document setting mvn heap size - trailing space

git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/trunk@1524651 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-HADOOP-9291-trunk-gd