Modify documentation to note sed "-E" option, now in POSIX, for EREs.

2013-10-16  David A. Wheeler <dwheeler@dwheeler.com>

	* doc/sed-in.texi: Document "-E" option to use EREs.
	  GNU sed already supported "-E" as an undocumented synonym
	  for "-r", but "-E" is now in POSIX.
	  See: http://austingroupbugs.net/view.php?id=528.
	* doc/sed.1: Likewise.
	* doc/sed.texi: Likewise.
	* doc/sed.x: Likewise.
	* sed/sed.c: Likewise.