Documentation should be more clear about time zones

[lesinigo]

The v2.10.4 (current as of this writing) documentation about TimePeriod objects does not explicitly details the "ranges" format and this issue is specifically about the lack of details about time zones.

While I hope that TimePeriods will one day support time zones (see #5374), it would be good if the documentation could be more clear about time zones handling:

• tell the reader that the "ranges" are expressed in local time, as handled by the operating system standard libraries
• tell the reader when time ranges are evaluated and if / how they account for time zone changes

Example scenario:

• I have a time range that says 07:30 to 00:00
• a little before midnight Icinga2 runs the last check of the day, the next one should be after 07:30 local time
• but between 00:00 and 07:30 we get a time zone change (DST starts / ends)
• does the interaction between the TimePeriod and the check scheduler and other parts work as expected and will I get the next check run around 07:30 on the "new" localtime, or maybe Icinga2 scheduled it with the old localtime and it will get run one hour earlier / one hour later?