libstdc++: Fix incorrect Docbook links

libstdc++: Fix incorrect Docbook links

The <xref> element creates the link text automatically from the link
target, rather than using the text node child of the element. This can
be changed by using an endterm attribute, but it's simpler to just use
the <link> element instead.

	* doc/xml/manual/containers.xml: Replace <xref> with <link>.
	* doc/xml/manual/evolution.xml: Likewise.
	* doc/html/manual/api.html: Regenerate.
	* doc/html/manual/containers.html: Regenerate.