[tz] Proposed patch - Theory notes for backward file

[Stephen Colebourne]

Theory notes for backward file

Note that the file exists for compatibility and canonicalization
Explicitly note IDs in that file are deprecated
Explicitly note that data set should make sense without backward
----------------
https://github.com/jodastephen/tz/commit/709e51dc7aeea8c2286b8a50c0d91d475b50d293

The aim of this patch is simple - to document what the backward file
is for in more detail.

Stephen
----------------

@@ -362,12 +362,19 @@ constraints.  Although a 'zone.tab' location's
longitude corresponds
 to its LMT offset with one hour for every 15 degrees east longitude,
 this relationship is not exact and is not true for 'time.tab'.

+The file 'backward' consists of Link entries mapping deprecated names
+to their replacements. Usage of a deprecated name is discouraged.
+As such, the file permits canonicalization, where the use of a
+deprecated name can be automatically replaced with the replacement.
+If the data set is used with the 'backward' file file excluded, then
+it must remain logical and complete. For example, there must be no
+references to any deprecated name from any other file.
+
 Older versions of this package used a different naming scheme,
-and these older names are still supported.
-See the file 'backward' for most of these older names
+and these older names are still supported using 'backward'.
 (e.g. 'US/Eastern' instead of 'America/New_York').
-The other old-fashioned names still supported are
-'WET', 'CET', 'MET', and 'EET' (see the file 'europe').
+Some other old-fashioned names are not deprecated, such as
+'WET', 'CET', 'MET', and 'EET', see the 'europe' file.