Make text-quoting-style customizable. Introduce t and new meaning for nil.
A value of nil for text-quoting-style now means "no translation". t means
"Use curved quotes if displayable".
* src/doc.c (text-quoting-style (function)): modify for new semantics.
(text-quoting-style (variable)): Amend the doc string, set the default value
to t.
* lisp/cus-start.el: (top level): Create a customize entry for
text-quoting-style in group display.
* etc/NEWS: Amend the entry for text-quoting-style.
* doc/emacs/display.texi (Text Display): Describe the translation of ASCII
quotes to curved quotes, and how to influence or inhibit it.
* doc/lispref/help.texi (Keys in Documentation): Change text-quoting-style
from a variable to a user option. Describe its changed set of values. State
that it can be customized freely.