IMPALA-3504: [DOCS] Document utc_timestamp()
IMPALA-3504: [DOCS] Document utc_timestamp()

This function seems to be related to
unix_micros_to_utc_timestamp() and
utc_to_unix_micros() which are also not
currently documented. Did they all come in
at the same time or have the *unix_micros*
ones existed for a while?

Change-Id: Ia2662fc79d588f22a24a5067429a57b3c0d0f0f0
Reviewed-on: http://gerrit.cloudera.org:8080/8190
Reviewed-by: Greg Rahn <67c1931175b8ff649ca2bf375786e47a4218c86d@cloudera.com>
Reviewed-by: Lars Volker <4d73c29a4561048ce85fa0051bb7a6d201781159@cloudera.com>
Tested-by: Impala Public Jenkins

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