Update datetime timezone handling after fix

This patch fixes Tarantool output on using `datetime.new()` and
`datetime.set()` with timezones specified.

After tarantool/tarantool#10970 datetime objects created/modified with
timestamps plus timezones are handled differently from the way they are
described in the doc. Now setting timezone only changes the local time
of day represented by the datetime object instead of affecting the
timestamp.

PR tarantool/tarantool#10970
Issue tarantool/tarantool#10363

Author: georgiy-belyanin

doc/reference/reference_lua/datetime.rst

@@ -221,7 +221,7 @@ Functions
                     tz = 'Europe/Moscow'
                     }
         ---
-        - 2022-07-01T08:30:05.122999906 Europe/Moscow
+        - 2022-07-01T11:30:05.122999906 Europe/Moscow
         ...
 
         tarantool> datetime.new {
@@ -230,7 +230,7 @@ Functions
                     tz = 'Europe/Moscow'
                     }
         ---
-        - 2022-07-01T08:30:05.000000123 Europe/Moscow
+        - 2022-07-01T11:30:05.000000123 Europe/Moscow
         ...
 
 
@@ -713,7 +713,7 @@ datetime_object
 
             tarantool> dt:set {tzoffset = 60}
             ---
-            - 2021-08-20T18:25:20.567+0100
+            - 2021-08-20T16:25:20.567+0100
             ...