Makefile: correct DC_SHA1 documentation

Makefile: correct DC_SHA1 documentation

The claim that DC_SHA1 takes priority over other *_SHA1 knobs was true
when it was added in [1], But that hasn't been the case since it was
made the fallback default in [2].

We should be making it not only the default, but something that takes
priority over other *_SHA1 knobs, but that's outside the scope of this
change. For now let's correct the documentation to match reality.

Let's also remove the "unconditionally enable" wording, per the above
the enabling of "DC_SHA1" is conditional on these other flags.

The "Define DC_SHA1" here is also a lie, actually it's "we don't care
if you define DC_SHA1, just don't define anything else", but that's a
more general issue that'll be addressed in a subsequent commit. Let's
first stop pretending that this setting (which we actually don't even
use) takes priority over anything else.

1. 8325e43b82d (Makefile: add DC_SHA1 knob, 2017-03-16)
2. e6b07da2780 (Makefile: make DC_SHA1 the default, 2017-03-17)

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