releases/15.0R/relnotes: Document AMD GPU slowness and VM domainset fixes

Replace the LinuxKPI paragraph talking only about handling the
__GFP_NORETRY flag in linux_alloc_pages() with the user-facing reason of
this work and an overview of other fixes towards this goal.

Add a separate paragraph about VM domainset iterator fixes, which were
prompted by the previous but are much larger in scope. One commit is
common to both paragraphs.