Add string-chop-newline

* doc/lispref/strings.texi (Creating Strings): Document it.
* lisp/emacs-lisp/subr-x.el (string-chop-newline): Add new function.

lisp/emacs-lisp/shortdoc.el

@@ -181,6 +181,8 @@ There can be any number of :example/:result elements."
   (string-remove-prefix
    :no-manual t
    :eval (string-remove-prefix "foo" "foobar"))
+  (string-chop-newline
+   :eval (string-chop-newline "foo\n"))
   (string-clean-whitespace
    :eval (string-clean-whitespace " foo   bar   "))
   (string-fill