[PATCH v4 6/9] 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 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 870ecdb0d85..992eba9e0c8 100644
--- a/Makefile
+++ b/Makefile
@@ -513,7 +513,15 @@ include shared.mak
 #
 # === SHA-256 backend ===
 #
-# Define BLK_SHA256 to use the built-in SHA-256 routines.
+# ==== Default SHA-256 backend ====
+#
+# If no *_SHA256 backend is picked we'll fall fall back on using the
+# default.
+#
+# The default SHA-256 backend is shipped with Git. No flag is required
+# to enable it. To select it don't define any other *_SHA256 flag.
+#
+# ==== Other SHA-256 implementations ====
 #
 # Define NETTLE_SHA256 to use the SHA-256 routines in libnettle.
 #
-- 
2.38.0.1251.g3eefdfb5e7a