Improve support of the Tai-Viet script

* lisp/language/tai-viet.el ("TaiViet"): Update the doc
string.  Suggested by Jim Brase <jim_brase@sil.org>.
(Bug#5806)

* etc/HELLO: Add a Tai Viet entry.

etc/HELLO

@@ -79,7 +79,9 @@ Spanish (espa</x-charset><x-charset><param>latin-iso8859-1</param>ñol)	¡Hola!
 Swedish (svenska)	Hej / Goddag / Hallå
 </x-charset><x-charset><param>mule-unicode-0100-24ff</param>Tamil (தமிழ்)	வணக்கம்
 Telugu (తెలుగు)	నమస్కారం
-</x-charset><x-charset><param>thai-tis620</param>Thai (ภาษาไทย)	สวัสดีครับ / สวัสดีค่ะ
+</x-charset>TaiViet (ꪁꪫꪱꪣ ꪼꪕ)	ꪅꪰꪙꫂ ꪨꪮꫂ ꪁꪫꪱ / ꪅꪽ ꪨꪷ ꪁꪫꪱ
+
+<x-charset><param>thai-tis620</param>Thai (ภาษาไทย)	สวัสดีครับ / สวัสดีค่ะ
 </x-charset><x-charset><param>tibetan</param>Tibetan (བོད་སྐད་)	བཀྲ་ཤིས་བདེ་ལེགས༎
 </x-charset><x-charset><param>mule-unicode-0100-24ff</param>Tigrigna (ትግርኛ)	ሰላማት
 </x-charset><x-charset><param>latin-iso8859-9</param>Turkish (Türkçe)	Merhaba

lisp/language/tai-viet.el

@@ -39,21 +39,20 @@
 	      (input-method . "tai-sonla")
 	      (sample-text . "TaiViet (ꪁꪫꪱꪣ ꪼꪕ)\t\tꪅꪰꪙꫂ ꪨꪮꫂ ꪁꪫꪱ / ꪅꪽ ꪨꪷ ꪁꪫꪱ")
 	      (documentation . "\
-TaiViet refers to the Tai language used by Tai people in
-Vietnam, and also refers to the script used for this language.
-Both the script and language have the same origin as that of Thai
+TaiViet refers to the Tai script, which is used to write several
+Tai languages of northwestern Vietnam and surrounding areas.  These
+languages are Tai Dam (also known as Black Tai or Tai Noir),
+Tai Dón (also known as White Tai or Tai Blanc), Tày Tac,
+Tai Daeng (also known as Red Tai or Tai Rouge),
+and Thai Song (also known as Lao Song).  However, some people
+consider Tai Dam, Tai Dón and Tai Daeng to be dialects of the
+same language, and call them collectively \"Tai Viet\".
+
+Both the script and languages have the same origin as that of Thai
 language/script used in Thailand, but now they differ from each
 other in a significant way (especially the scripts are).
 
 The language name is spelled as \"ꪁꪫꪱꪣ ꪼꪕ\", and the script name is
-spelled as \"ꪎ ꪼꪕ\" in the modern form, \"ꪎꪳ ꪼꪕ\" in the traditional
-form.
-
-As the proposal for TaiViet script to the Unicode is still on
-the progress, we use the Private Use Area for TaiViet
-characters (U+F000..U+F07E).  A TaiViet font encoded accordingly
-is available at this web page:
-    http://www.m17n.org/viettai/
-")))
+spelled as \"ꪎꪳ ꪼꪕ\".")))
 
 (provide 'tai-viet)