Makefile: rephrase the discussion of *_SHA1 knobs

In the preceding commit the discussion of the *_SHA1 knobs was left
as-is to benefit from a smaller diff, but since we're changing these
let's use the same phrasing we use for most other knobs. E.g. "define
X", not "define X environment variable", and get rid of the "when
running make to link with" entirely.

Furthermore the discussion of DC_SHA1* options is now under a "Options
for the sha1collisiondetection implementation" heading, so we don't
need to clarify that these options go along with DC_SHA1=Y, so let's
rephrase them accordingly.

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

Makefile

@@ -492,20 +492,20 @@ include shared.mak
 # Define DC_SHA1 to enable the collision-detecting sha1
 # algorithm. This is slower, but may detect attempted collision attacks.
 #
-# Define BLK_SHA1 environment variable to make use of the bundled
-# optimized C SHA1 routine.
+# Define BLK_SHA1 to make use of optimized C SHA-1 routines bundled
+# with git (in the block-sha1/ directory).
 #
-# Define OPENSSL_SHA1 environment variable when running make to link
-# with the SHA1 routine from openssl library.
+# Define OPENSSL_SHA1 to link to the SHA-1 routines from the OpenSSL
+# library.
 #
 # ==== Options for the sha1collisiondetection library ====
 #
-# Define DC_SHA1_EXTERNAL in addition to DC_SHA1 if you want to build / link
+# Define DC_SHA1_EXTERNAL if you want to build / link
 # git with the external SHA1 collision-detect library.
 # Without this option, i.e. the default behavior is to build git with its
 # own built-in code (or submodule).
 #
-# Define DC_SHA1_SUBMODULE in addition to DC_SHA1 to use the
+# Define DC_SHA1_SUBMODULE to use the
 # sha1collisiondetection shipped as a submodule instead of the
 # non-submodule copy in sha1dc/. This is an experimental option used
 # by the git project to migrate to using sha1collisiondetection as a