doc/cpp: Document __has_include_next

While hacking on an unrelated change, I noticed that __has_include_next
hasn't been documented at all.  This patch adds it to the __has_include
manual node.

gcc/ChangeLog:

	* doc/cpp.texi (__has_include): Document __has_include_next
	also.
	(Conditional Syntax): Mention __has_include_next in the
	description for the __has_include menu entry.