dc_datetime function documentation

[Kristaps Dzonsons]

> Enclosed is a patch that documents all of the dc_datetime functions.
> 
> The CAVEATS are important because calling gmtime et al from a library
> might not be expected.  (They touch the zoneinfo files, which may not be
> available on embedded or sandboxed systems.)
> 
> I also specifically note the differences to "struct tm" in terms of the
> year and month field.

Ignore previous.

Resent with dc_datetime_gmtime and dc_datetime_localtime updated to
reflect that "result" is filled in and what happens if "result" is NULL.

Best,

Kristaps
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