Make the value nil in text-quoting-style mean what it does in Emacs 25.
This is a partial reversion of yesterday's commit by the same author, which
changed the meaning of nil and introduced the new value t.
* src/doc.c (text_quoting_style, text-quoting-style)
(internal--text-quoting-flag): Revert yesterday's changes.
* lisp/cus-start.el: (top level): Amend the entry for text-quoting-style.
* etc/NEWS: Amend the entry for text-quoting-style.
* doc/lispref/control.texi (Signalling Errors)
* doc/lispref/display.texi (Displaying Messages)
* doc/lispref/strings.texi (Formatting Strings): Bind text-quoting-style to
grave rather than nil to inhibit translation of quotes.
* doc/lispref/help.texi (Keys in Documentation): Revert the description of the
proposed new default, t.