docs: Fix up new instances of index reordering

This commit fixes up an instance of the index entry mis-ordering that
occurred between the formulation and application of commit
r13-6310-gf33d7a88d069d1.

gcc/ChangeLog:

	* doc/extend.texi: Associate use_hazard_barrier_return index
	entry with its attribute.
	* doc/invoke.texi: Associate -fcanon-prefix-map index entry with
	its attribute