added documentation for Dates.TimeZone, Dates.UTC

see JuliaLang#31202

Author: mrdofu

stdlib/Dates/docs/src/index.md

@@ -643,6 +643,8 @@ Dates.TimeType
 Dates.DateTime
 Dates.Date
 Dates.Time
+Dates.TimeZone
+Dates.UTC
 ```
 
 ## Dates Functions

stdlib/Dates/src/types.jl

@@ -83,7 +83,20 @@ abstract type Calendar <: AbstractTime end
 # ISOCalendar provides interpretation rules for UTInstants to civil date and time parts
 struct ISOCalendar <: Calendar end
 
+"""
+    TimeZone
+
+Geographic zone generally based on longitude determining what the time is at a certain location.
+Some time zones observe daylight savings (eg EST -> EDT).
+"""
 abstract type TimeZone end
+
+"""
+    UTC
+
+`UTC`, or Coordinated Universal Time, is the [`TimeZone`](@ref) from which all others are measured.
+It is associated with the time at 0° longitude. It is not adjusted for daylight savings.
+"""
 struct UTC <: TimeZone end
 
 """