(set-language-info): Doc-string

(set-language-info): Doc-string
describes `coding-priority' KEY.
(set-language-environment-coding-systems): New function.
(list-subset-p): New function.
(select-safe-coding-system): New function.
(set-language-info): New optional args DESCRIBE-MAP and SETUP-MAP.
(set-language-info-alist): New optionla arg PARENTS.  Call
set-language-info with apropriate DESCRIBE-MAP and SETUP-MAP args.
(set-language-environment-coding-systems): New function.

(prefer-coding-system): Call update-iso-coding-systems.