Clarify documentation of cpu_dispatch/cpu_specific

There has been some internal confusion lately as to how cpu_dispatch and
cpu_specific dispatch to processors, so this patch clarifies the
documentation to make it more clear that:

1- Unlike ICC, we do not consider the vendor string (that is, an AMD
processor might result in something other than generic)

2- there are some processors that aren't really distinguishable thanks
to the library limitation, so the variant being selected is unspecified.

In reality, I believe the 'stable_sort' makes it so the 1st one that
meets the requirements is the one that is selected (and that matches my
experimented result), but I don't want to limit our implementation.