document start_time format strings (#581)

Signed-off-by: Derek Argueta <dereka@pinterest.com>

docs/root/configuration/access_log.rst

@@ -36,6 +36,12 @@ The following command operators are supported:
   TCP
     Downstream connection start time including milliseconds.
 
+  START_TIME can be customized using a `format string <http://en.cppreference.com/w/cpp/io/manip/put_time>`_, for example:
+
+.. code-block:: none
+
+  %START_TIME(%Y/%m/%dT%H:%M:%S%z %s)%
+
 %BYTES_RECEIVED%
   HTTP
     Body bytes received.

docs/root/intro/version_history.rst

@@ -7,6 +7,7 @@ Version history
   <arch_overview_load_balancing_types_round_robin>` support. The round robin
   scheduler now respects endpoint weights and also has improved fidelity across
   picks.
+* access log: ability to format START_TIME
 * admin: added ``/stats/prometheus`` as an alternative endpoint for getting stats in prometheus format.
 * logger: all :ref:`logging levels <operations_admin_interface_logging>` can be configured
   at run-time: trace debug info warning error critical.