Document datetime.parse

[TarantoolBot]

We support relaxed ISO-8601/RFC339 date formats. Consider RFC-3339 as a
profile of a strict ISO-8601, not major extension. We use c-dt, which is
ISO-8601 conformant in such way that we accept superset of ISO-8601
strings, with a few reasonable extensions.

Few examples for further clarification:

Years

Basic	Extended	...	ISO-8601?
20121224	2012-12-24	Calendar date	yes
2012359	2012-359	Ordinal date	yes
2012W521	2012-W52-1	Week date	yes
2012Q485	2012-Q4-85	Quarter date

Modified c-dt parser supports extended years
values, which are outside of standard [0001 .. 9999] range

Tarantool extended values
0000-01-01	-0001-12-31
5879611-07-11	-5879610-06-22

Time

Basic	Extended
T12	N/A
T1230	T12:30
T123045	T12:30:45
T123045.123456789	T12:30:45.123456789
T123045,123456789	T12:30:45,123456789

The time designator T may be omitted. This is exactly how RFC3339 is
different to strict ISO-8601.

Timezone

We do not (yet) support names for standard time-zones (like MSK, or
EST, or America/New_York or whatever) we only support numeric offsets
at the moment.

In a form

Basic	Extended
Z	N/A
+hh	N/A
-hhmm	-hh:mm
GMT+h
GMT-hhmm
`GMT+h:mm	GMT+hh:mm
UTC-h:mm	UTC-hh:mm

And relaxed ISO-8601 format is compositions of all relaxed subformats (i.e.
date, time, and timezone) putting them together as:

{relaxed-date} ( [Tt ] {relaxed-time} ( ' ' {lenient-time-zone})? )?

NO_TEST=documentaton update
Closes #6731
Requested by @tsafin in tarantool/tarantool@9bc059b.

[patiencedaur]

Duplicate of #2576. Information copied to the tracked issue. Closing.