Improve documentation of string truncation APIs

Improve documentation of string truncation APIs

* doc/lispref/display.texi (Size of Displayed Text):
* lisp/international/mule-util.el (truncate-string-to-width):
Document caveats of using 'truncate-string-to-width' when
character composition is involved.

* lisp/emacs-lisp/subr-x.el (string-limit):
* doc/lispref/strings.texi (Creating Strings): Improve the
documentation of 'string-limit'.