Doc and manual fixes related to mule

Doc and manual fixes related to mule

* doc/emacs/mule.texi (International): Copyedits.
(International Chars): Update C-x = example output.
(Disabling Multibyte): Rename from "Enabling Multibyte".
Clarify what "unibyte: t" does, and mode-line description.
(Unibyte Mode): Update for "Disabling Multibyte" node name change.
Use Texinfo recommended convention for quotes+punctuation.

* doc/emacs/custom.texi (Specifying File Variables):
Fix "unibyte" description.
Update for "Disabling Multibyte" node name change.

* doc/emacs/emacs.texi: Update for "Disabling Multibyte" node name change.

* doc/lispref/loading.texi (Loading Non-ASCII):
"unibyte:" can also be at the end.

* lisp/international/mule.el (set-auto-coding-for-load): Doc fix.