IMPALA-5512: [DOCS] Distinguish GROUP BY and ORDER BY in SELECT syntax
IMPALA-5512: [DOCS] Distinguish GROUP BY and ORDER BY in SELECT syntax

Turned one instance of GROUP BY in the syntax declaration
into ORDER BY. Moved NULLS FIRST and NULLS LAST from first
instance of GROUP BY to the new ORDER BY line.

Change-Id: I9df24cf5af4f2b0aabdb388b4e46a75e0e5275a7
Reviewed-on: http://gerrit.cloudera.org:8080/7789
Reviewed-by: Alex Behm <fe1626037acfc2dc542d2aa723a6d14f2464a20c@cloudera.com>
Tested-by: Impala Public Jenkins

cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala,cloudera/Impala