dc_datetime function documentation

[Kristaps Dzonsons]

> Looks good, except for one thing:
> 
>> 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.)
> 
> Why would that be unexpected? They are standard C library functions. I
> see no reason not to use them (and there are basically no easy
> alternatives). If their implementation is broken on some system, then
> that's a libc problem. So if you ask me, this caveats section is not
> necessary and only adds confusion.

Jef,

It's not that libc is broken w/r/t these functions, but it should be
noted that they're being used to specify how localtime is being computed.

(Why?  First, because according to POSIX, localtime_r might invoke
tzset() beforehand, or it might not.  Moreover, in chroots or in
capabilities environments, localtime might behave like gmtime due to
file-system restrictions.)

Perhaps this should be in an IMPLEMENTATION NOTES section instead of the
CAVEATS section, however, because it's not a bad thing --- just
clarification.

(For safety, it should also be noted in dc_parser_get_datetime that the
dc_datetime_localtime function might be invoked.)

I forgot to edit Makefile.am in my patch.  If you'd like, I can rephrase
the CAVEAT as an IMPLEMENTATION NOTE and amend the Makefile.am in
another patch?

Best,

Kristaps