pgsql: Add comments about additional mule-internal charsets from emacs'

Add comments about additional mule-internal charsets from emacs's
source code(lisp/international/mule-conf.el). These charsets have not
been supported up to now anyway, so this is just for adding
commentary. Also add mention that we follow emacs's implementation,
not xemacs's.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/1c7a7faa5b74042dc3b3726296a17ff913dcd84c

Modified Files
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src/include/mb/pg_wchar.h | 14 +++++++++++---
1 files changed, 11 insertions(+), 3 deletions(-)