Pacify some docstring control char warnings

Pacify some docstring control char warnings

Other instances are discussed in the following thread:
https://lists.gnu.org/r/emacs-devel/2024-02/msg00797.html

* lisp/allout.el (allout-command-prefix): Declare :type as
key-sequence.  Mark up key sequences in docstring.
* lisp/auth-source.el (auth-source--decode-octal-string):
* lisp/ffap.el (ffap-search-backward-file-end):
* lisp/gnus/gnus-art.el (gnus-page-delimiter):
* lisp/gnus/nnheader.el (nnheader-strip-cr):
* lisp/proced.el (proced-log):
* lisp/progmodes/idlw-shell.el (idlwave-shell-prompt-pattern):
* lisp/url/url-http.el (url-http-clean-headers):
* lisp/vcursor.el (vcursor-interpret-input): Quote control
characters in docstrings.