The Doxygen docs do not have stable URLs, so linking to specific pages
doesn't work well.

	* doc/xml/manual/backwards_compatibility.xml: Remove link to
	Doxygen-generated pages with unstable URL.
	* doc/xml/manual/concurrency_extensions.xml: Likewise.
	* doc/xml/manual/extensions.xml: Likewise.
	* doc/xml/manual/parallel_mode.xml: Likewise.
	* doc/xml/manual/support.xml: Likewise.

From-SVN: r269881