Redo text-quoting-style variable

Redo text-quoting-style variable

Rename help-quote-translation to text-quoting-style,
and use symbols rather than characters as values.
This follows suggestions along these lines by Alan Mackenzie in:
http://lists.gnu.org/archive/html/emacs-devel/2015-06/msg00343.html
and by Drew Adams in:
http://lists.gnu.org/archive/html/emacs-devel/2015-08/msg00048.html
* doc/lispref/help.texi (Keys in Documentation)
* etc/NEWS:
* lisp/cus-start.el (standard):
* src/doc.c (Fsubstitute_command_keys, syms_of_doc):
Document and/or implement the new behavior instead of the old.
(syms_of_doc): New symbols 'grave' and 'straight'.