[PATCH v5 06/10] Makefile: document default SHA-256 backend

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Since 27dc04c5450 (sha256: add an SHA-256 implementation using
libgcrypt, 2018-11-14) we've claimed to support a BLK_SHA256 flag, but
there's no such SHA-256 backend.

Instead we fall back on adding "sha256/block/sha256.o" to "LIB_OBJS"
and adding "-DSHA256_BLK" to BASIC_CFLAGS.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 251368b315d..646fbe5b7dd 100644
--- a/Makefile
+++ b/Makefile
@@ -515,14 +515,15 @@ include shared.mak
 #
 # ==== SHA-256 implementations ====
 #
-# Define BLK_SHA256 to use the built-in SHA-256 routines.
-#
 # Define NETTLE_SHA256 to use the SHA-256 routines in libnettle.
 #
 # Define GCRYPT_SHA256 to use the SHA-256 routines in libgcrypt.
 #
 # Define OPENSSL_SHA256 to use the SHA-256 routines in OpenSSL.
 #
+# If don't enable any of the *_SHA256 settings in this section, Git
+# will default to its built-in sha256 implementation.
+#
 # == DEVELOPER defines ==
 #
 # Define DEVELOPER to enable more compiler warnings. Compiler version
-- 
2.38.0.1464.gea6794aacbc