Correct DST transitions for Taiwan in 1946, 1947, and 1979.

* asia (Asia/Taipei):
DST in 1946 was adopted from May 15 through Sep 30.
DST in 1947 was adopted form Apr 15 through Oct 31.
DST in 1979 was adopted from Jul 1 through Sep 30.
* asia (Asia/Taipei): 1945 transition was 09-21 01:00, not 10-25.

Also, update commentary to describe recent changes better.
(Thanks to Yu-Cheng Chuang.)
* NEWS: Document this.