Update and fix Time documentation

[aaronfranke]

Follow-up to #49123 and #54256.

• Fix bogus docs in get_datetime_string_from_system (my fault, bogus since Add a Time singleton #49123).
• Fix and clarify statements about UTC added in Docs: Add some notes about timezones and UTC to Time #54256. "Unix epoch assumes UTC" as a blanket statement is false. Time's conversion methods are timezone-agnostic, and when getting information from the system there is usually a utc bool.
• Fix some style issues with time methods in OS having parameters named utc instead of p_utc.

[mhilbrunner]

Thanks!