doc: convert some TeX accents to UTF-8

doc: convert some TeX accents to UTF-8

* doc/emacs/ack.texi (Acknowledgments):
* doc/emacs/emacs.texi (Acknowledgments):
* doc/misc/emacs-mime.texi (Interface Functions):
Convert some TeX accents (e.g., '@l{}') to UTF-8 (e.g., 'ł').
Apparently the TeX accents cause problems when generating gnu.org
web pages, e.g., @l{} is rendered as '/l' on
<http://www.gnu.org/software/emacs/manual/html_node/
emacs/Acknowledgments.html>.