ARROW-13218: [Doc] Document/clarify conventions for timestamp storage

I've made an attempt to refine the recent discussions into an updated comment describing the timestamp column.  Since the entire discussion has been around fine-grained semantic concepts I will appreciate even minor suggestions to improve the wording.  There are still votes ongoing so this shouldn't be merged until those resolve.

Closes apache#10629 from westonpace/feature/ARROW-13218

Authored-by: Weston Pace <[email protected]>
Signed-off-by: Benjamin Kietzman <[email protected]>

format/Schema.fbs

@@ -218,8 +218,33 @@ table Time {
 /// leap seconds, as a 64-bit integer. Note that UNIX time does not include
 /// leap seconds.
 ///
-/// The Timestamp metadata supports both "time zone naive" and "time zone
-/// aware" timestamps. Read about the timezone attribute for more detail
+/// Date & time libraries often have multiple different data types for temporal
+/// data.  In order to ease interoperability between different implementations the
+/// Arrow project has some recommendations for encoding these types into a Timestamp
+/// column.
+///
+/// An "instant" represents a single moment in time that has no meaningful time zone
+/// or the time zone is unknown.  A column of instants can also contain values from
+/// multiple time zones.  To encode an instant set the timezone string to "UTC".
+///
+/// A "zoned date-time" represents a single moment in time that has a meaningful
+/// reference time zone.  To encode a zoned date-time as a Timestamp set the timezone
+/// string to the name of the timezone.  There is some ambiguity between an instant
+/// and a zoned date-time with the UTC time zone.  Both of these are stored the same.
+/// Typically, this distinction does not matter.  If it does, then an application should
+/// use custom metadata or an extension type to distinguish between the two cases.
+///
+/// An "offset date-time" represents a single moment in time combined with a meaningful
+/// offset from UTC.  To encode an offset date-time as a Timestamp set the timezone string
+/// to the numeric time zone offset string (e.g. "+03:00").
+///
+/// A "local date-time" does not represent a single moment in time.  It represents a wall
+/// clock time combined with a date.  Because of daylight savings time there may multiple
+/// instants that correspond to a single local date-time in any given time zone.  A
+/// local date-time is often stored as a struct or a Date32/Time64 pair.  However, it can
+/// also be encoded into a Timestamp column.  To do so the value should be the the time
+/// elapsed from the Unix epoch so that a wall clock in UTC would display the desired time.
+/// The timezone string should be set to null or the empty string.
 table Timestamp {
   unit: TimeUnit;
 
@@ -232,11 +257,9 @@ table Timestamp {
   /// Whether a timezone string is present indicates different semantics about
   /// the data:
   ///
-  /// * If the time zone is null or equal to an empty string, the data is "time
-  ///   zone naive" and shall be displayed *as is* to the user, not localized
-  ///   to the locale of the user. This data can be though of as UTC but
-  ///   without having "UTC" as the time zone, it is not considered to be
-  ///   localized to any time zone
+  /// * If the time zone is null or an empty string, the data is a local date-time
+  ///   and does not represent a single moment in time.  Instead it represents a wall clock
+  ///   time and care should be taken to avoid interpreting it semantically as an instant.
   ///
   /// * If the time zone is set to a valid value, values can be displayed as
   ///   "localized" to that time zone, even though the underlying 64-bit