Makefile: document SHA-1 and SHA-256 default and selection order

Makefile: document SHA-1 and SHA-256 default and selection order

For the *_SHA1 and *_SHA256 flags we've discussed the various flags,
but not the fact that when you define multiple flags we'll pick one.

Which one we pick depends on the order they're listed in the Makefile,
which differed from the order we discussed them in this documentation.

Let's be explicit about how we select these, and re-arrange the
listings so that they're listed in the priority order we've picked.

I'd personally prefer that the selection was more explicit, and that
we'd error out if conflicting flags were provided, but per the
discussion downhtread of[1] the consensus was to keep theses semantics.

This behavior makes it easier to e.g. integrate with autoconf-like
systems, where the configuration can provide everything it can
support, and Git is tasked with picking the first one it prefers.

1. https://lore.kernel.org/git/220710.86mtdh81ty.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>