datetime_object: specify supported iso8601 formats

[TarantoolBot]

Related dev. issue(s): https://github.com/tarantool/tarantool/pull/6731/files#r829826291

Product: Tarantool
Since: 2.10
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/datetime/datetime_object/#datetime-parse
SME: @ tsafin @ ligurio

Details

<…>ime information.
format (string) – indicator of the input_sting |format|. Possible values: ‘iso8601’, ‘rfc3339’, or strptime-like format<…>

https://www.tarantool.io/en/doc/latest/reference/reference_lua/datetime/datetime_object/#datetime-parse

We don't support all possible iso8601 formats, see discussion in https://github.com/tarantool/tarantool/pull/6731/files#r829826291

Definition of done

• the reference contains clear and precise information about supported datetime formats

[ligurio]

See also:

• Document datetime.parse #2792
• tarantool/tarantool@c6c2def