Document curved quotes a bit better

Document curved quotes a bit better

* doc/emacs/basic.texi (Inserting Text):
Mention C-x 8.  Change example to use curved quote rather
than infinity, as this lets us give more ways to do it.
* doc/emacs/mule.texi (International Chars): Mention C-x 8 shortcuts
and quotation marks.
* doc/emacs/text.texi (Quotation Marks):
* doc/lispref/tips.texi (Documentation Tips):
Add "curly quotes" and "curved quotes" to the index.
* doc/emacs/text.texi (Quotation Marks):
Give the C-x 8 shorthands for curved quotes.
Cross-reference to "Quotation Marks".