Document ALTER TABLE ... ADD IF NOT EXISTS PARTITION

Users aren't aware of ALTER TABLE ... ADD IF NOT EXISTS:

https://groups.google.com/a/cloudera.org/forum/#!topic/impala-user/QhDJ5yT8CRk

https://groups.google.com/a/cloudera.org/forum/#!topic/impala-user/6LPuX5DzMF0

It seems like it should be documented here:

http://www.cloudera.com/content/www/en-us/documentation/enterprise/latest/topics/impala_alter_table.html