pgsql: Improve documentation about MULE encoding.

Improve documentation about MULE encoding.

This commit improves the comments in pg_wchar.h and creates #define symbols
for some formerly hard-coded values.  No substantive code changes.

Tatsuo Ishii and Tom Lane

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/09022de1f55708bcfe8b2bf206d818055c34d49e

Modified Files
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.../euc_tw_and_big5/euc_tw_and_big5.c              |   17 +--
src/backend/utils/mb/wchar.c                       |    6 +
src/include/mb/pg_wchar.h                          |  145 +++++++++++++-------
3 files changed, 110 insertions(+), 58 deletions(-)