Revert markup change in with-coding-priority docs

Revert markup change in with-coding-priority docs

This partially reverts commit fc759eb9b3
"Fix with-coding-priority markup in Elisp manual"
of 2019-10-13T15:36:02Z!contovob@tcd.ie.

For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-10/msg00550.html
https://lists.gnu.org/archive/html/emacs-devel/2020-06/msg00473.html

* doc/lispref/nonascii.texi (Specifying Coding Systems): Use more
specific cross-reference to progn even if info.el displays it
suboptimally.