Avoid non-ASCII decoding errors in Texinfo files

Avoid non-ASCII decoding errors in Texinfo files

* doc/misc/tramp.texi:
* doc/lispref/strings.texi:
* doc/lispref/positions.texi:
* doc/lispref/help.texi:
* doc/lispref/functions.texi:
* doc/lispintro/emacs-lisp-intro.texi:
* doc/emacs/text.texi:
* doc/emacs/modes.texi:
* doc/emacs/mini.texi:
* doc/emacs/display.texi:
* doc/emacs/custom.texi:
* doc/emacs/basic.texi: Add 'coding' cookies -- these files use
Unicode characters and should be decoded as UTF-8.
* doc/lispref/frames.texi (Size Parameters): Don't use a non-ASCII
apostrophe unnecessarily.