Escape ` and ' in doc

Escape ` and ' in doc

* gnus-art.el (gnus-article-address-banner-alist)
(gnus-face-properties-alist):
* gnus-sum.el (gnus-newsgroup-variables):
* nndiary.el (nndiary-reminders):
* nnmail.el (nnmail-fancy-expiry-targets, nnmail-split-fancy):
Escape apostrophes and grave accents in docstrings if they
are supposed to stand for themselves and are not quotes.  Remove
apostrophes from docstring examples like ‘'(calendar-nth-named-day
-1 0 10 year)’ that confuse source code with data.  Do some other
minor docstring fixups as well, e.g., insert a missing close
quote.