(Language Environments): Explain how to update the X

(Language Environments): Explain how to update the X
server's font database after installing Intlfonts.
(Language Environments): Add rationale for using the language
environment hook.
(Select Input Method): Add an example of setting
default-input-method inside set-language-environment-hook.
(Recognize Coding): Explain who would like to disable EOL
conversions.
(Specify Coding): Add an index entry for non-ASCII file names.